[FPEnv] Teach the IRBuilder about constrained FPTrunc and FPExt

The IRBuilder doesn't know that FPTrunc and FPExt have constrained
equivalents. Add 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/D64934

llvm-svn: 366477