Regression was introduced in
https://swiftshader-review.googlesource.com/c/SwiftShader/+/41269

When detecting and removing intermediate stores, if the storeValue is a
constant, we skip it; however, we must make sure to set unmatchedLoads
to true, or the optimizer will incorrectly eliminate the skipped store
on a later iteration.

This bug lead to incorrect generated code, which failed certain deqp-vk
tests, e.g. dEQP-VK.clipping.user_defined.clip_cull_distance.vert.2_6

Bug: b/148272103
Change-Id: I024d75decdc4f1d13e7f82cade50f8be09d68f3e
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/41888Tested-by: Antonio Maiorano <amaiorano@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>