Debug printing of MIR is poor, which makes it difficult to debug the more
complex Wasm->MIR translations, mostly because so much information is missing.

This patch improves the basic printed structure and adds a framework by which
we can incrementally add printing of MIR-node-specific fields in future. The
following changes are made:

• Printing of MIR expressions in nested style is removed. This is confusing
  in that casual reading of the output suggests that expressions are sometimes
  evaluated twice. This is removed and replaced with standard non-nested
  printing of (SSA) operands.

• The result type of each MIR node is now printed before its name. This helps
  in debugging wasm 32-vs-64 bit code, for example.

• Block IDs are now printed, so they can be referred to by branch insns.

• A generic mechanism for printing extra, node-specific, fields has been
  introduced: overrideable method MDefinition::getExtras. By default this
  does nothing, but individual MIR classes can override it to provide any
  extra info they want.

• For a few classes, MDefinition::getExtras has been filled in: for
  constants, comparisons, branch instructions, and fixed offsets in some wasm
  memory access insns.

Here's a before and after example showing all the above improvements:

Before:

  --
  36:abs (18:sub (12:phi) (17:add))
  37:wasmfloatconstant
  38:compare (36:abs (18:sub)) (37:wasmfloatconstant)
  39:test (38:compare (36:abs) (37:wasmfloatconstant))
  --
  40:wasmtruncatetoint32 (18:sub (12:phi) (17:add))
  41:goto
  --
  42:constant 0x80000000
  43:goto
  --
  45:wasmbinarybitwise (44:phi (40:wasmtruncatetoint32) (42:constant 0x80000000)) (31:constant 0x1)
  46:wasmreturn (45:wasmbinarybitwise (44:phi) (31:constant 0x1)) (2:wasmparameter)

After:

  Block4:
    36 = Float32.abs 18
    37 = Float32.wasmfloatconstant 2.147484e+09:f32
    38 = Int32.compare ty=Float32 jsop=Lt 36 37
    39 = None.test true->Block5 false->Block6 38
  Block5:
    40 = Int32.wasmtruncatetoint32 18
    41 = None.goto Block7
  Block6:
    42 = Int32.constant 0x80000000
    43 = None.goto Block7
  Block7:
    44 = Int32.phi 40 42
    45 = Int32.wasmbinarybitwise And 44 31
    46 = None.wasmreturn 45 2

Debug printing of MIR is poor, which makes it difficult to debug the more
complex Wasm->MIR translations, mostly because so much information is missing.

This patch improves the basic printed structure and adds a framework by which
we can incrementally add printing of MIR-node-specific fields in future. The
following changes are made:

• Printing of MIR expressions in nested style is removed. This is confusing
  in that casual reading of the output suggests that expressions are sometimes
  evaluated more than once. This is removed and replaced with standard
  non-nested printing of (SSA) operands.

• The result type of each MIR node is now printed before its name. This helps
  in debugging wasm 32-vs-64 bit code, for example.

• Block IDs are now printed, so they can be referred to by branch insns.

• A generic mechanism for printing extra, node-specific, fields has been
  introduced: overrideable method MDefinition::getExtras. By default this
  does nothing, but individual MIR classes can override it to provide any
  extra info they want.

• For a few classes, MDefinition::getExtras has been filled in: for
  constants, comparisons, branch instructions, and fixed offsets in some wasm
  memory access insns.

The only tricky bit is the ExtrasCollector class that is used to collect up
the (C) strings created by the ::getExtras method. This needs to

(1) ensure that those methods can safely pass on stack-allocated strings, and

(2) ensure that none of those methods calls the system malloc/free directly --
this would be an embedding bug.

ExtrasCollector solves these by respectively (1) copying any strings it is
given and (2) using js::SystemAllocPolicy as the allocator both for the
string copies and the vector that holds them.

https://hg.mozilla.org/mozilla-central/rev/2383d2a30a60