Prior to this change, the SPIR-V would be transformed as part of
GlslangGetShaderSpirvCode() and then possibly another time afterward to
disable the early fragment test optmization (for monolithic programs) or
to update the descriptor set/binding values (for PPOs). By storing the
original SPIR-V in the ProgramVk and the transformed SPIR-V in the
ProgramExecutableVk, we can now remove the SPIR-V transformation from
GlslangGetShaderSpirvCode() and only perform it once in
ProgramInfo::initProgram() for monolithic programs and PPOs.

Bug: angleproject:4513
Test: CQ
Change-Id: I4e7240b2abe7c796c46cf531948dad0e37c1419a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2267537
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>