optimizeSingleBasicBlockLoadsStores() supersedes both
eliminateLoadsFollowingSingleStore() and
optimizeStoresInSingleBasicBlock().

This was verified by adding asserts the latter when the delete more
instructions. They're never hit by dEQP-VK tests. Only the
PointerToPointer triggers an assert. It goes away when running
optimizeSingleBasicBlockLoadsStores() twice. Since it is very rare to
store the address of a pointer in another pointer, and before the new
optimization pass was implemented we also did not handle this case,
we're not going to pay the cost of running it twice.

eliminateUnitializedLoads() was moved earlier because we never produce
new loads of uninitialized values in the optimization passes and it's
best to eliminate these allocas early.

The late eliminateDeadCode() was also removed since we already run it
at the end of optimizeSingleBasicBlockLoadsStores().

TODOs for cases where the load/store sizes don't match have been removed
because propagating a different type causes issues.

Bug: b/179668593
Change-Id: I4b3533057423709cdaa8343301184d8225b0727b
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/53128
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>