Previously any SPIR-V OpExecutionMode instruction would override the
execution mode parameters, so a shader module with multiple entry points
with different execution modes would not work as intended.

Bug: b/155550449
Change-Id: I86211e66a3cef5a3bbf9e535e9be8415b5d69d4b
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/45249
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>