[turbofan] support for Int64 in CheckedLoad/CheckedStore on 64-bit platforms.

[turbofan] support for Int64 in CheckedLoad/CheckedStore on 64-bit platforms.

This is to support WebAssembly 64-bit ints in the short term, since it
currently uses CheckedLoad/CheckedStore for accesses to the memory. In the
long run, we'll change this to be explicit bounds checks that throw on out
of bounds.

R=bmeurer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1310323006

Cr-Commit-Position: refs/heads/master@{#30555}