[builtins] Divide shouldn't underflow if rounded result would be normal.

We were treating certain edge cases that are actually normal as denormal
results, and flushing them to zero; we shouldn't do that. Not sure this
is the cleanest way to implement this edge case, but I wanted to avoid
adding any code on the common path.

Differential Revision: https://reviews.llvm.org/D59070

llvm-svn: 356529