[PowerPC] Map max/minnum intrinsics and fmax/fmin to ISD nodes for CTR-based loop legality

Intrinsic::maxnum and Intrinsic::minnum, along with the associated libc
function calls (fmax[f], etc.) generally map to function calls after lowering.
For some vector types with QPX at least, however, we can legally lower these,
and we don't need to prohibit CTR-based loops on their account.

It turned out, however, that the logic that checked the opcodes associated with
intrinsics was broken (it would set the Opcode variable, but that variable was
later checked only if set for some otherwise-external function call.

This fixes the latter problem and adds the FMAX/MINNUM mappings.

llvm-svn: 264532