[PowerPC] Change method names; NFC

Changed method names based on the discussion in https://reviews.llvm.org/D34986:
getInt64 -> selectI64Imm,
getInt64Count -> selectI64ImmInstrCount.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@309541 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed