Replace SPIR-V assertion with mixed-types error message.

This is very unlikely to occur in real-world code, as it's somewhat
nonsense to use the comma operator in this way. However, it's better to
fail cleanly than to assert.

Change-Id: I76481cd8a993cb1a798ee16956400a512efd4c15
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/337636
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
diff --git a/src/sksl/SkSLSPIRVCodeGenerator.cpp b/src/sksl/SkSLSPIRVCodeGenerator.cpp
index 7d67e9c..e4b8390 100644
--- a/src/sksl/SkSLSPIRVCodeGenerator.cpp
+++ b/src/sksl/SkSLSPIRVCodeGenerator.cpp
@@ -2198,7 +2198,7 @@
             }
             return result;
         } else {
-            SkASSERT(false);
+            fErrors.error(leftType.fOffset, "unsupported mixed-type expression");
             return -1;
         }
     } else {