UBOs can be significantly larger than our constant register file. Don't
clamp indexes into them.

We *should* catch accesses outside the bound region of the UBO as well,
but that's a preexisting bug (consider a UBO region smaller than the
constant register file), and we don't have the limit available here.

Bug: b/145227408
Change-Id: I67aae6ba9a10290c1b69587808e274c481925078
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38708
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Chris Forbes <chrisforbes@google.com>