Use a C array for temporary load results

Use a C array for temporary load results

Array<> emits GEP instructions on every access, which aren't necessary.
Note that these temporaries are required because the If/Else blocks
both write to them, while the destination intermediate object is an
rvalue which can only be initialized once to maintain SSA form.

Also EmitAccessChain was moved to match the declaration order.

Bug b/128539387

Change-Id: I726fb0fd28b0a19a61e8759679e30bc699f3279d
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/27788
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>