only emit _imm ops when JITing for x86

There are probably ways to make this more efficient by only optimizing
what's necessary (e.g. try JIT first, then interpreter only if it fails)
and some other performance improvements to make, but for now I want to
focus mostly on keeping things simple and correct.

The line between Builder::done() and Program::Program() is particularly
fuzzy and becoming fuzzier here, and I think that'll be something
that'll change eventually.

This makes SkVMTest debug dumps more portable, though perhaps less
useful.  Might kill that feature soon now that SkVM is tested more
thoroughly in unit tests and GMs and bots and such.

Change-Id: Id9ce8daaf8570e5bea8b10f1a80b97f5b33d45dc
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/269941
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
4 files changed