[spirv-reader] Propagate depth conversions through functions.

When converting the depth information, we were attempting to convert the
variables before propagation through functions. This caused an assert
when we tried to get the root variable and instead got a function
parameter.

This CL updates the reader to handle the propagation through functions
for depth information before attempting to do the substitutions.

Change-Id: I30ec35d03bfc347c86d98c1c7fa48708eee505b8
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/288515
Reviewed-by: James Price <jrprice@google.com>
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Auto-Submit: dan sinclair <dsinclair@chromium.org>
Commit-Queue: James Price <jrprice@google.com>