This CL introduces a ProtectedLoad instruction with is needed for out
of bounds trap handling. ProtectedLoad behaves like a regular load,
but it takes a context and source position parameter as well. These
are used by an out of line code fragment to generate code to throw a
JS exception for an out of bounds memory reference in Wasm.

These changes a cleaned up subset of
https://codereview.chromium.org/2148743004/

The rest of this feature will follow in future CLs. This includes a
table mapping memory instructions to landing pads as well as the
actual signal handler.

BUG= https://bugs.chromium.org/p/v8/issues/detail?id=5277

Review-Url: https://codereview.chromium.org/2301833004
Cr-Commit-Position: refs/heads/master@{#39318}