[LangRef] make it clear that FP instructions do not have side effects

Also, fix the undef vs. UB example to use 'sdiv' because that can trigger div-by-zero UB.

The existing text for the constrained intrinsics says:
"By default, LLVM optimization passes assume that the rounding mode is round-to-nearest 
and that floating point exceptions will not be monitored. Constrained FP intrinsics are 
used to support non-default rounding modes and accurately preserve exception behavior 
without compromising LLVM’s ability to optimize FP code when the default behavior is 
used."
...so the additional text with the normal FP opcodes should make the different modes
clear.

Differential Revision: https://reviews.llvm.org/D44216

llvm-svn: 327138