X87: [RegisterConfiguration] Streamline access to arch defaults, simplify Registers.

  port 257336d2 (r37297)

  original commit message:
  Replaces ArchDefault method with Crankshaft and Turbofan getters.
  Eliminates IsAllocated method on Register, FloatRegister, DoubleRegister.
  Eliminates ToString method too.
  Changes call sites to access appropriate arch default RegisterConfiguration.

BUG=

Review-Url: https://codereview.chromium.org/2114553002
Cr-Commit-Position: refs/heads/master@{#37413}

src/x87/code-stubs-x87.h

@@ -298,8 +298,8 @@ class RecordWriteStub: public PlatformCodeStub {
                                   Register r2,
                                   Register r3) {
       for (int i = 0; i < Register::kNumRegisters; i++) {
-        Register candidate = Register::from_code(i);
-        if (candidate.IsAllocatable()) {
+        if (RegisterConfiguration::Crankshaft()->IsAllocatableGeneralCode(i)) {
+          Register candidate = Register::from_code(i);
           if (candidate.is(ecx)) continue;
           if (candidate.is(r1)) continue;
           if (candidate.is(r2)) continue;