On 64 bit architectures, reading memory in wasm requires a conversion
of the 32 bit index into a 64 bit value. For this, we generate a
ChangeUint32ToUint64 graph node, which later materializes as a move
and hence consumes a register. This generates unneccesary register
pressure, especially with gvn enabled.

Now, the WasmGraphBuilder will emit a IntPtrConstant node right
away if the index is a constant.

Change-Id: Id8d7efa90c6bd8e4d7b6419e2075802dfc1eb383
Reviewed-on: https://chromium-review.googlesource.com/1099061
Commit-Queue: Stephan Herhut <herhut@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53726}