[PPC64] Optimize redundant instructions in global access sequences.

The access sequence for global variables in the medium and large code models use
2 instructions to add an offset to the toc-pointer. If the offset fits whithin
16-bits then the instruction that sets the high 16 bits is redundant.

This patch adds the --toc-optimize option, (on by default) and enables rewriting
of 2 instruction global variable accesses into 1 when the offset from the
TOC-pointer to the variable (or .got entry) fits in 16 signed bits. eg

addis %r3, %r2, 0           -->     nop
addi  %r3, %r3, -0x8000     -->     addi %r3, %r2, -0x8000

This rewriting can be disabled with the --no-toc-optimize flag

Differential Revision: https://reviews.llvm.org/D49237

llvm-svn: 342602