PPC: remove unused WriteInt32ToHeapNumberStub.
Fix alignment of Code::kConstantPoolOffset.
PPC: prefer li vs. constant pool load when no relocation.
PPC: perf enhancement: remove sign extension for non-overflow integer add/sub.

Conflicts:

	src/ppc/lithium-codegen-ppc.cc
PPC: perf enhancement: prefer isel over branch logic.

Conflicts:

	src/globals.h
	src/ppc/lithium-codegen-ppc.cc
PPC: fix simulation of isel instruction.
PPC: perf enhancement: additional isel opportunities.

Conflicts:

	src/ppc/code-stubs-ppc.cc
PPC: perf enhancement: optimize sign extension logic.

Conflicts:

	src/ppc/lithium-codegen-ppc.cc
	src/ppc/macro-assembler-ppc.cc
PPC: perf enhancement: improve overflow checks.

Conflicts:

	src/ppc/lithium-codegen-ppc.cc
PPC: Remove hard-coded assumptions about Smi/HeapObject tag values.
PPC: perf enhancement: modify 64-bit object tagging scheme.

... such that tagged pointers are 4-byte aligned.  This allows
loads/stores of pointer-sized fields to be done via a single
instruction on Power platforms.  Previously 1-byte aligned tagged
pointers required a two instruction sequence to first align the
pointer and then access memory using an adjusted 4-byte aligned
immediate offset.
Work around gcc-4.6.1 compiler warning.