[FPEnv][AArch64] Add lowering and instruction selection for strict conversions

Strict fp-to-int and int-to-fp conversions can be handled in the same way that
the non-strict versions are (by using the appropriate instruction or converting
to a function call when we have no instruction).

Differential Revision: https://reviews.llvm.org/D73625

(cherry picked from commit 0bb9a27c)