Fix textureSize for non-uniform samplers.

Fix textureSize for non-uniform samplers.

GLSL textureSize() calls that use a sampler that is not a uniform,
should not use the index of the register as the texture unit index.
Instead the index is the value stored in the register itself.

Bug chromium:904265

Change-Id: Iee1058e789daae15248ae5ffa940a0155dca61b5
Reviewed-on: https://swiftshader-review.googlesource.com/c/22910
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>