Return undef on FP <-> Int conversions that overflow (PR21330).
The LLVM Lang Ref states for signed/unsigned int to float conversions:
"If the value cannot fit in the floating point value, the results are undefined."
And for FP to signed/unsigned int:
"If the value cannot fit in ty2, the results are undefined."
This matches the C definitions.
The existing behavior pins to infinity or a max int value, but that may just
lead to more confusion as seen in:
Returning undef will hopefully lead to a less silent failure.
Differential Revision: http://reviews.llvm.org/D5603