Don't fold double constant to an integer if dest type not integral

Summary:
I encountered this issue when constant folding during inlining tried to
fold away a bitcast of a double to an x86_mmx, which is not an integral
type. The test case exposes the same issue with a smaller code snippet
during early CSE.

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D18528

llvm-svn: 265367