Deterministic loops use the first scalar of the SIMD register used as
the loop index, for addressing arrays. This means that operations on the
index register should not be masked (i.e. it should be treated as a
scalar).

Previously we were still masking it based on conditional statements, and
we didn't disable the masking altogether for the loop initialization and
initial test. A new shader assembly instruction 'SCALAR' was added for
doing this.

Previously this was conflated with the 'TEST' instruction, which should
independently disable/restore the 'continue' mask.

Bug swiftshader:93
Bug b/118009174

Change-Id: I4add1a6d74231f463217e57adfabdc81faf489ae
Reviewed-on: https://swiftshader-review.googlesource.com/c/22348Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>