[PowerPC] Remove redundant load immediate instructions

Currently PowerPC backend emits code like this:

  r3 = li 0
  std r3, 264(r1)
  r3 = li 0
  std r3, 272(r1)

This patch fixes that and other cases where a register already contains a value that is loaded so we will get:

  r3 = li 0
  std r3, 264(r1)
  std r3, 272(r1)

Differential Revision: https://reviews.llvm.org/D64220

llvm-svn: 366840