Attributes that are not active shouldn't have made it to the
ShaderInterfaceVariableInfoMap of gl::ShaderType::Vertex.  This change
turns a few ifs to ASSERT in SpirvVertexAttributeAliasingTransformer.

Bug: angleproject:4249
Change-Id: I3da4ea773eae20fe2f126b82f7790e9f2cab0bc1
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2491929Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>