Fix handling of special and large vals in expand pattern for `round`

The current expand pattern for `math.round` does not handle the
special values -0.0, +-inf, and +-nan correctly. It also does not
properly handle values with magnitude |x| >= 2^23. Lastly, the pattern
generates invalid IR when the input to `math.round` is a vector. This
patch fixes these issues.

Reviewed By: rsuderman

Differential Revision: https://reviews.llvm.org/D148398