[CodeGen] Round [SU]INT_TO_FP result when promoting from f16.

If we don't, values that aren't precisely representable in f16 could
be used as-is in a promoted f32 operation, which would produce
incorrect results.

AArch64 had the correct behavior; add a focused test.

Fixes http://llvm.org/PR26871

llvm-svn: 268700