[GlobalISel][AArch64] Make FP constraint checks consider possible use/def banks

In a few places in getInstrMapping, we check if use/def instructions for the
instruction we're mapping have floating point constraints.

We can improve this check and reduce the number of copies in GISel-compiled code
if we make a couple observations:

- For a def instruction, it only matters if the def instruction must always
  output a value stored on a FPR

- For a use instruction, it only matters if the use instruction must always
  only take in values stored in FPRs

This adds two new functions:

- onlyUsesFP
- onlyDefinesFP

Then we can use those when we're checking the uses/defs instead.

Without this patch, the load, unmerge, store, and select in the added test
would have unnecessary copies.

Differential Revision: https://reviews.llvm.org/D62426

llvm-svn: 361679