[FPEnv][X86] More strict int <-> FP conversion fixes

Fix several several additional problems with the int <-> FP conversion
logic both in common code and in the X86 target. In particular:

- The STRICT_FP_TO_UINT expansion emits a floating-point compare. This
  compare can raise exceptions and therefore needs to be a strict compare.
  I've made it signaling (even though quiet would also be correct) as
  signaling is the more usual default for an LT. This code exists both
  in common code and in the X86 target.

- The STRICT_UINT_TO_FP expansion algorithm was incorrect for strict mode:
  it emitted two STRICT_SINT_TO_FP nodes and then used a select to choose one
  of the results. This can cause spurious exceptions by the STRICT_SINT_TO_FP
  that ends up not chosen. I've fixed the algorithm to use only a single
  STRICT_SINT_TO_FP instead.

- The !isStrictFPEnabled logic in DoInstructionSelection would sometimes do
  the wrong thing because it calls getOperationAction using the result VT.
  But for some opcodes, incuding [SU]INT_TO_FP, getOperationAction needs to
  be called using the operand VT.

- Remove some (obsolete) code in X86DAGToDAGISel::Select that would mutate
  STRICT_FP_TO_[SU]INT to non-strict versions unnecessarily.

Reviewed by: craig.topper

Differential Revision: https://reviews.llvm.org/D71840