Currently all conditional branches in SPIR-V shaders are 'flattened',
resulting in the execution of all instructions in both code paths, even
in the case where none of the SIMD lanes are active. If a logically
never taken code path contains an image sampling instruction which
accesses a sampler or image array out of bounds, this could lead to
reading invalid descriptor data, or a page fault.

This is fixed by checking if any SIMD lanes are active, and if not,
jumping over the actual sampling operations which access the
descriptor data.

Bug: b/153380916
Change-Id: I8e0cf7ef855e1250ce6dce9ebf7a47e29da7814d
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43549Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>