[wasm][cleanup] Use Int32/64Constant() in WasmGraphBuilder

These functions are defined in WasmGraphBuilder, so we do not need to
invoke them in mcgraph() or gasm_.

Change-Id: Ibd97add56d22bff7eb99addd78bf599057b21041
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2717302Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73001}