[AArch64][GlobalISel] Assign FPR banks to loads which are used by integer->float conversions.

G_[US]ITOFP users of loads on AArch64 can operate on both gpr and fpr banks for scalars.
Because of this, if their source is a load, then that load can be assigned to an fpr
bank and therefore avoid having to do a cross bank copy via a gpr->fpr conversion.

Differential Revision: https://reviews.llvm.org/D94701