[VM] Introduce new way to define instruction backends (Reland).

Rewrite most SIMD instructions on ARM using this new way.

Our current way for defining instruction backends -- a pair of two virtual
methods called MakeLocationSummary and EmitNativeCode, leads to unnecessary
duplicated and verbose code. Code generation happens in three steps:

1. For each instruction in the graph MakeLocationSummary is called to
constructing a location summary object encoding register allocation constraints;
2. When all register constraints are collected a register allocation is performed
and results are filled back into the location summaries;
3. For each instruction in the graph EmitNativeCode is called. It unpacks
location summary attached to the instruction into actual machine registers and
emits native code.

There is usually a lot of duplication between declaring register constraints in
MLS and unpacking them in ENC which this CL is trying to remove.

The new way is centered on the concept of an *emitter function* which encodes
in its signature register constraints for a particular instruction.

We use a combination of templates and macroses to enable writing

    DEFINE_BACKEND(BinaryFloat32x4Op,
                   (QRegister result, QRegister left, QRegister right)) {
      // ...
    }

Instead of

    LocationSummary* BinaryFloat32x4OpInstr::MakeLocationSummary(Zone* zone,
                                                                 bool opt) const {
      const intptr_t kNumInputs = 2;
      const intptr_t kNumTemps = 0;
      LocationSummary* summary = new (zone)
          LocationSummary(zone, kNumInputs, kNumTemps, LocationSummary::kNoCall);
      summary->set_in(0, Location::RequiresFpuRegister());
      summary->set_in(1, Location::RequiresFpuRegister());
      summary->set_out(0, Location::RequiresFpuRegister());
      return summary;
    }

    void BinaryFloat32x4OpInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
      const QRegister left = locs()->in(0).fpu_reg();
      const QRegister right = locs()->in(1).fpu_reg();
      const QRegister result = locs()->out(0).fpu_reg();
      // ...
    }

This change also introduces a new, more handy way to work with S/D components of QRegisters, QRegister_ wrapper type.

Bug: #30949
Change-Id: I7bb0fc9672c89acc3d3d9b5e9859ae5a5471f420
Reviewed-on: https://dart-review.googlesource.com/11820
Commit-Queue: Martin Kustermann <[email protected]>
Reviewed-by: Martin Kustermann <[email protected]>

Author: mraleph