[AArch64][GlobalISel] Change G_FCONSTANTs feeding into stores into G_CONSTANTS

Given the following situation:

x = G_FCONSTANT (something that can't be materialized)
G_STORE x, some_addr

We know that x must be materialized as at least a single mov. However, at the
time of selection, the G_STORE will have been regbankselected to a FPR store.

So, as a result, you'll get an unnecessary fmov into the G_STORE.

Storing a constant value in a GPR and a constant value in a FPR are the same.
So, whenever you see a G_FCONSTANT that feeds into only G_STORES, so might as
well make it a G_CONSTANT.

This adds a target-specific combine which changes G_FCONSTANTs feeding into
G_STOREs into G_CONSTANTs.

Differential Revision: https://reviews.llvm.org/D72814