[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