[InstCombine] fadd double (sitofp x), y check that the promotion is valid

Doing these transformations check that the result of integer addition is representable in the FP type.

(fadd double (sitofp x), fpcst) --> (sitofp (add int x, intcst))
(fadd double (sitofp x), (sitofp y)) --> (sitofp (add int x, y))

This is a fix for https://bugs.llvm.org//show_bug.cgi?id=27036

Reviewed By: andrew.w.kaylor, scanon, spatel

Differential Revision: https://reviews.llvm.org/D31182

llvm-svn: 301018