[ARM] Teach getIntImmCostInst about the cost of saturating fp converts

Given a min(max(fptosi, INT_MIN), INT_MAX) with the correct constants,
we can now generate a fptosi.sat. But in the arm backend, the constant
can be treated as high cost, pulling it out of the basic block in a way
that the DAG combine can no longer see it. This teaches it again that it
is a low cost constant, not worth hoisting out.

Recommitted from 0e98659e with a fix for APInt comparison.

Differential Revision: https://reviews.llvm.org/D114380