[PATCH] Return undef on conversions that overflow (PR21330).

[Sanjay Patel]

Hi asl, hfinkel, sylvestre.ledru,

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:
http://llvm.org/bugs/show_bug.cgi?id=21130

Returning undef will hopefully lead to a less silent failure.

http://reviews.llvm.org/D5603

Files:
  lib/IR/ConstantFold.cpp
  test/Transforms/InstCombine/cast.ll
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