Optimize @switch statements in SwitchStatement::Make.

At IR generation time, this CL limits our optimizations to only
@switch statements. A regular switch statement will only be optimized
during the optimization phase even if the switch-value is a known
compile-time constant. This is done to avoid upsetting our reachability
analysis.

Most of this CL is moving existing logic from SkSLCompiler into
SkSLAnalysis and SkSLSwitchStatement. Although the diffs look large, the
actual changes are very small.

Change-Id: I90920f41bc386dfa7a980ae7510f6681231a5120
Bug: skia:11340, skia:11342, skia:11319
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/372679
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
9 files changed