Allow compile-time intrinsic evaluation to read const variables.

Previously, the code neglected to resolve constant variables into
values. This meant that expressions like `lessThan(zero, one)` could not
be compile-time evaluated even when `zero` and `one` have known values.

Change-Id: I2f5ce303e3dcc682be14e4d2485e24dd7c59212e
Bug: skia:10835
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/405536
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
9 files changed