[PowerPC] Expand ISEL instruction into if-then-else sequence.

[PowerPC] Expand ISEL instruction into if-then-else sequence.

Generally, the ISEL is expanded into if-then-else sequence, in some
cases (like when the destination register is the same with the true
or false value register), it may just be expanded into just the if
or else sequence.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@292128 91177308-0d34-0410-b5e6-96231b3b80d8