Convert IRGenerator::convertPrefixExpr to PrefixExpression::Make.

Surprisingly, this actually improved our error detection slightly.
The expression `- -half4(0)` can now be simplified to `half4(0)` at
IR generation time, which allows the constant-folder to detect a
constant zero (and from there, a division by constant zero).

Change-Id: I8c4f6ab522efab5bf98913f9c6a1487b7af39a99
Bug: skia:11342, skia:11343
Auto-Submit: John Stiles <>
Reviewed-by: Brian Osman <>
8 files changed