Implement gather/scatter operations for shader register files.

Implement gather/scatter operations for shader register files.

This allows to address the registers with a vector of indices.

Also rename 'dynamic' register files to 'indirect addressable', to
disambiguate from 'dynamic indexing' at the shader level. Indexing with
a uniform does not require gather/scatter operations, but does require
indirect addressing.

Bug chromium:845103
Bug skia:7846

Change-Id: I3c42be33def66328688f2900c61c80246bf1e584
Reviewed-on: https://swiftshader-review.googlesource.com/18989
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>