Yet another horrible phi/loop edge case (pun intended).

Added test.

Bug: b/133440380
Bug: b/133481698
Change-Id: I327842fa2d4314bce938454da81f67f890cf9e12
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/31845
Presubmit-Ready: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>