[PowerPC] Enhance the fast selection of cmp instruction and clean up related asserts

Fast selection of llvm icmp and fcmp instructions is not handled well about VSX instruction support.

We'd use VSX float comparison instruction instead of non-vsx float comparison instruction 
if the operand register class is VSSRC or VSFRC because i32 and i64 are mapped to VSSRC and 
VSFRC correspondingly if VSX feature is opened.

If the target does not have corresponding VSX instruction comparison for some type, 
just copy VSX-related register to common float register class and use non-vsx comparison instruction.

Differential Revision: https://reviews.llvm.org/D57078

llvm-svn: 352174

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