[X86] Cleanup convertIntLogicToFPLogic a little. NFCI

-Use early returns to reduce indentation
-Replace multipe ifs with a switch.
-Replace an assert with an llvm_unreachable default in the switch.
-Check that the FP type we're going to use for the
 X86ISD::FAND/FOR/FXOR is legal rather than checking that the
 integer type matches the width of a legal scalar fp type. This all
 runs after legalization so it shouldn't really matter, but making
 sure we're using a valid type in the X86ISD node is really
 whats important.

llvm-svn: 362565