Avoid rounding intermediate values when the intermediate value is not
consumed. For example, this avoids rounding the return value of
assignment, so "b = a;" becomes "b = frm(a);" instead of "frm(b =
frm(a))".

BUG=angle:874
TEST=compiler_tests

Change-Id: Ifcdb53fb1d07a2cf24e429cc237c2d0262dc32f8
Reviewed-on: https://chromium-review.googlesource.com/241852Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>