Fix diagonal-matrix assertion discovered by fuzzer.

This was another place where we needed to use
`getConstantSubexpression` to rebuild vectors/matrices; it is a more
robust approach than trying handle each ctor type individually. The
fuzzer found an edge case with double-casting matrices to vectors that
fell through the cracks with the original approach.

In adding additional tests, I also found a case that the constant-folder
seems to ignore, `bool4(x,x,x,x) == bool4(x)`. This does fold for ints
and floats, so this ought to be fixable in a followup, but it's not a
big deal either way; this is very unlikely to occer in real code.

Change-Id: I4d577c87ef7049306685ca95250ecdf93b1dbc06
Bug: oss-fuzz:37464
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/441238
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
7 files changed