[AArch64][GlobalISel] Infer whether G_PHI is going to be a FPR in regbankselect

Some instructions (G_LOAD, G_SELECT, G_UNMERGE_VALUES) check if their uses
will define/use FPRs (using `onlyUsesFP` and `onlyDefinesFP`).

The register bank of a use isn't necessarily known when an instruction asks for
this.

Teach `hasFPConstraints` to look at the instructions feeding into a G_PHI when
its destination bank is unknown. If any of them are FPR, assume the entire
G_PHI will also be assigned a FPR.

Since a phi can have many inputs, and those inputs can in turn be phis,
restrict the search depth to a very low number.

Also improve the docs for `hasFPConstraints` and friends a little.

This is a 0.3% code size improvement on CTMark/Bullet at -O3, and a 0.2% code
size improvement at CTMark/pairlocalalign at -O3.

Differential Revision: https://reviews.llvm.org/D88177