A cast like `float(five)` or `int4(colorGreen)` should detect const
variables and replace the expression with its compile-time constant
equivalent value. At present, this replacement is missed, which inhibits
further optimization opportunities on the expression.
(This CL is very similar in spirit to http://review.skia.org/404676)
Change-Id: I04b5c435a30d2afcdbdb3d020adc15e9c651cc31
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/405682
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
