[GlobalISel][AArch64] Legalize vector G_FPOW

This instruction is legalized in the same way as G_FSIN, G_FCOS, G_FLOG10, etc.

Update legalize-pow.mir and arm64-vfloatintrinsics.ll to reflect the change.

Differential Revision: https://reviews.llvm.org/D60218

llvm-svn: 358764