[FPEnv] Teach the IRBuilder about constrained FPToSI and FPToUI.

The IRBuilder doesn't know that the two floating point to integer instructions
have constrained equivalents. This patch adds the support by building on
the strict FP mode now present in the IRBuilder.

Reviewed by:	John McCall
Approved by:	John McCall
Differential Revision:	https://reviews.llvm.org/D67291

llvm-svn: 371235