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: prefer li vs. constant pool load when no relocation.
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