[AArch64][GlobalISel] Add selection support for fpr bank source variants of G_SITOFP and G_UITOFP.

In order to import patterns for these, we need to define new ops that can map to
the AArch64ISD::[SU]ITOF nodes. We then transform fpr->fpr variants of the generic
opcodes to these custom opcodes in preisel-lowering. We have to do it here and
not the PostLegalizer combiner because this has to run after regbankselect.

Differential Revision: https://reviews.llvm.org/D94702