Previously the pointee type of OpVariable was determined starting from
the result ID. It just obtains the 'Object' which has the type collected
from the OpVariable instruction itself. So the object lookup can be
skipped by using the latter directly again.

Bug b/129000021

Change-Id: I882d837c4cc1c7a8d0b19de38dc8ee19e622af33
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/27651
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>