Fix crash when swizzling a bvec with constant 0/1s.

Code like
  bool4 result = val.xy01;

Will now be converted to:
  bvec4 result = bvec4(val.xy, bool(0), bool(1));

Previously it tried to do this, but there isn't an implicit conversion
from int to bool, so it was silently failing and adding nulls into the
constructor:
  bvec4 result = bvec4(val.xy, $coerceToBool(0), $coerceToBool(1));

This CL also cleans up some related code that I was checking while
trying to understand the nature of the error.

Change-Id: I5b7d96760a03170ff78b46251c4182cc4e89836f
Bug: oss-fuzz:25781
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/318636
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
2 files changed