Previously SIMD::Load would assume that all pointers are 4-byte aligned.
However, texel pointers generated by SpirvShader::GetTexelAddress are
not aligned for small formats.

Add an alignment parameter to SIMD::Load and use it in EmitImageRead.

Bug: b/135954761
Change-Id: I6a420049e98f42a68960d557dee933fee9487af3
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/33328Tested-by: Chris Forbes <chrisforbes@google.com>
Presubmit-Ready: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>