With --trace-wasm-decoder, we were printing the local variable index
for values on the stack generated by get_local instructions. This CL
adds the same feature for globals.

R=titzer@chromium.org

Change-Id: Ie113ebcc2d1abcd67df2d01bf4bdb452635732c8
Reviewed-on: https://chromium-review.googlesource.com/684737Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48170}