Convert IRGenerator::convertPostfixExpr to PostfixExpr::Make.

We have no optimizations on postfix-expressions at all so this is a very
straightforward cut-and-paste.

Change-Id: I73b3bc73ad833e668306301e6d6859e44230393a
Bug: skia:11342
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/376846
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
8 files changed