Converted SkSLSwitchCase to use ints rather than expressions

There was no reason to carry around the heavier expressions, as we only
support int literals or constants, and this not only fixes a bug but
simplifies some of the code.

Bug: skia:12811
Change-Id: I3f55d6974d77ce1907f90868775c0e4894243783
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/494037
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
12 files changed