Signature indices are encoded as SMIs in the function table of a wasm
module. To compare a signature index with the index parameter that is
passed to an indirect function call, we now change the signature index
to an int32 instead of encoding the index parameter as a SmiConstant.
SmiConstants get encoded as NumberConstant nodes in the TF graph, which
can cause the allocation of heap numbers. Heap allocation has to be
avoided for parallel compilation.

R=bradnelson@chromium.org, titzer@chromium.org

Review-Url: https://codereview.chromium.org/2090923002
Cr-Commit-Position: refs/heads/master@{#37240}