ANGLE only collects inactive varyings by instance name. The SPIR-V
transformer expects I/O blocks to be identified by block name. If an I/O
block is inactive, an assertion would fire because the block name is not
in the shader variable info map.

Bug: angleproject:5676
Test: GLSLTest_ES31.VaryingIOBlockNotDeclaredIn*Shader*
Change-Id: I40e51c9770b03457e3896dcb2afa4e9525b098d8
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2742658Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>