Mark FMOV constant materialization as being as cheap as a move.

This prevents us from doing things like LICM'ing it out of a loop,
which is usually a net loss because we end up having to spill a
callee-saved FPR to accomodate it.

This does perturb instruction scheduling around this instruction,
so a number of tests had to be updated to account for it.

Reviewed By: t.p.northover

Differential Revision: https://reviews.llvm.org/D87316