[SimplifyLibCalls] Enable double to float shrinking for copysign

rdar://19049359

Differential Revision: http://reviews.llvm.org/D6495
[SimplifyLibCalls] Improve double->float shrinking to consider constants

This allows cases like float x; fmin(1.0, x); to be optimized to fminf(1.0f, x);

rdar://19049359

Differential Revision: http://reviews.llvm.org/D6496