[LangRef] describe the default FP environment

Follow-up for D44216: add a section and examples to describe the FP env.
Also, add pointers from the FP instructions to this new section to reduce
bloat.

Differential Revision: https://reviews.llvm.org/D44318


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@327998 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed