[ConstantFolding] Use FoldBitCast correctly

Previously we might attempt to use a BitCast to turn bits into vectors of pointers,
but that requires an inttoptr cast to be legal. Add an assertion to detect the formation of illegal bitcast attempts
early (in the tests, we often constant-fold away the result before getting to this assertion check),
while being careful to still handle the early-return conditions without adding extra complexity in the result.

Patch by Jameson Nash <jameson@juliacomputing.com>.

Differential Revision: https://reviews.llvm.org/D65057

llvm-svn: 372940