This will enable reusing it to determine the root node of rvalues as
well. The only functional change is that struct indexing no longer
overrides the register type. This is of no effect here since lvalue
intermediates already inherited their type from the root node, but for
rvalues the intermediates are considered temporary registers, while
instead the root's type should be used.

Change-Id: I2dbd1b0f8886c3f111a2ed3ef7fe4e9a5b480085
Reviewed-on: https://swiftshader-review.googlesource.com/13930Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>