[Statepoint] Constant fold FP deopt args.

We do not have any special handling for constant FP deopt arguments.
They are just spilled to stack or generated in register by MOVS
instruction. This is inefficient and, when we have too many such
constant arguments, may result in register allocation failure.
Instead, we can bitcast such constant FP operands to appropriately
sized integer and record as constant into statepoint and later, into
StackMap.

Reviewed By: skatkov
Differential Revision: https://reviews.llvm.org/D80318