[PowerPC] Remove assertion "Shouldn't overwrite a register before it is killed"

The assertion is everzealous and fail tests like:

  renamable $x3 = LI8 0
  STD renamable $x3, 16, $x1
  renamable $x3 = LI8 0

Remove the assertion since killed flag of $x3 is not mandentory.

Differential Revision: https://reviews.llvm.org/D68344

llvm-svn: 374515