[ARM] Add custom strict fp conversion lowering when non-strict is custom

We have custom lowering for operations converting to/from floating-point types
when we don't have hardware support for those types, and this doesn't interact
well with the target-independent legalization of the strict versions of these
operations. Fix this by adding similar custom lowering of the strict versions.

This fixes the last of the assertion failures in the CodeGen/ARM/fp-intrinsics
test, with the remaining failures due to poor instruction selection.

Differential Revision: https://reviews.llvm.org/D71127