Make uitofp and sitofp defined on overflow.

IEEE 754 defines the expected result on overflow. As far as I know,
hardware implementations (of f16), and compiler-rt (__floatuntisf)
correctly return +-Inf on overflow. And I can't think of any useful
transform that would take advantage of overflow being undefined here.

Differential Revision: https://reviews.llvm.org/D47807

llvm-svn: 334777