Move caller-saves saving/restoring to ReadBarrierMarkRegX.

Instead of saving/restoring live caller-save registers
before/after the call to read barrier mark entry points
ReadBarrierMarkRegX, have these entry points save/restore
all the caller-save registers themselves (except register
rX, which contains the return value).

Also refactor the assembly code of these entry points
using macros.

* Boot image code size variation on Nexus 5X
  (aosp_bullhead-userdebug build):
  - total ARM64 framework Oat files size change:
    119196792 bytes -> 115575920 bytes (-3.04%)
  - total ARM framework Oat files size change:
    100435212 bytes -> 97621188 bytes (-2.80%)

* Benchmarks (ARM64) score variations on Nexus 5X
  (aosp_bullhead-userdebug build):
  - RitzPerf (lower is better)
    - average score difference: -2.71%
  - CaffeineMark (higher is better)
    - no real difference for most tests
      (absolute variation lower than 1%)
    - better score on the "Method" benchmark:
      score variation 41253 -> 44891 (+8.82%)

Test: ART host and target (ARM, ARM64) tests.
Bug: 29506760
Bug: 12687968
Change-Id: I881bf73139a3f1c2bee9ffc6fc8c00f9a392afa6
diff --git a/compiler/optimizing/code_generator.h b/compiler/optimizing/code_generator.h
index 47e6625..5e6e175 100644
--- a/compiler/optimizing/code_generator.h
+++ b/compiler/optimizing/code_generator.h
@@ -80,7 +80,11 @@
 
   virtual void EmitNativeCode(CodeGenerator* codegen) = 0;
 
+  // Save live core and floating-point caller-save registers and
+  // update the stack mask in `locations` for registers holding object
+  // references.
   virtual void SaveLiveRegisters(CodeGenerator* codegen, LocationSummary* locations);
+  // Restore live core and floating-point caller-save registers.
   virtual void RestoreLiveRegisters(CodeGenerator* codegen, LocationSummary* locations);
 
   bool IsCoreRegisterSaved(int reg) const {