[PowerPC][LoopVectorize] Extend getRegisterClassForType to consider double and...

[PowerPC][LoopVectorize] Extend getRegisterClassForType to consider double and other floating point type

In https://reviews.llvm.org/D67148, we use isFloatTy to test floating
point type, otherwise we return GPRRC.
So 'double' will be classified as GPRRC, which is not accurate.

This patch covers other floating point types.

Reviewed By: #powerpc, nemanjai

Differential Revision: https://reviews.llvm.org/D71946