[PATCH] Fix for PR20059 (instcombine reorders shufflevector after instruction that may trap)

[hfinkel at anl.gov]

"but a system or user can certainly enable those manually" -- No, Clang/LLVM specifically do not support this! In the context of C, this means that we don't support "#pragma STDC FENV_ACCESS on". It is not well defined for the user to fool with the fp environment without informing the compiler (in C, that pragma is the standard mechanism), and if the user tries, they'll receive an error.

http://reviews.llvm.org/D4424