refactor Program building

Do less in Builder, more in Program::Program().

This temporarily disables JITs until I rewrite
them to also build from these new inputs:

   - vector<Builder::Instruction>
   - vector<Val> deaths
   - vector<int> strides

i.e. to do their own register assignment
and make their own hoisting decisions.

Change-Id: Ie2ce9755f20860a80506e913b7b139d562e291c3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/228216
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
2 files changed