Optimizer::eliminateLoadsFollowingSingleStore() was only looking at a
single instruction following the store.

Subzero derives Ice::Inst from llvm::ilist_node<Inst>, making it a node
in a circular linked list. However, we can't iterate until the end
because by default ilist_node<> does not provide sentinel node tracking.
If the llvm::ilist_node<Inst, llvm::ilist_sentinel_tracking<true>> class
is used instead, we can use isSentinel(), but this comes at the cost of
each node itself having to indicate whether it's the sentinel, and
keeping that up to date.

This change instead obtains the basic block's instruction list from the
store instruction, to get the iterator of the end node.

Bug: b/179668593
Change-Id: I994a7b37d8872380d2668c098d85000120d3e47a
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/52753
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>