Fix assertion when performing type-coercion with enums.

In http://review.skia.org/393397, I replaced a Constructor::Convert call
with a call directly to ConstructorCompoundCast::Make.

This worked fine if the input expression was actually a compound, but
if it was not, the code would assert/crash. The fuzzer detected this
error right away. (Enums are not considered to be a scalar, a vector or
a matrix in SkSL.)

Change-Id: Ie0df4c5771ff4f4d8f5251d4703e9c3516b6baad
Bug: oss-fuzz:33113
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/395720
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>
4 files changed