Enable SPIR-V compiler optimization (#4402)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4402

Call the SPIR-V compiler with the `-O` flag, which enables optimizations when compiling GLSL to SPIR-V. The `-Os` flag (which tries to minimize SPIR-V size) was tested as well, but resulted in (very) slightly worse performance.

Reviewed By: jorgep31415

Differential Revision: D60193514

fbshipit-source-id: 2dfb999fb1951a63a990773ab563a1a3a3c304b0