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This patch adds alias set annotations to the MIR generated for
struct.{new,set,get}, so as to enable the existing GVN machinery to remove
duplicate loads of the OOL block pointer in the (static) presence of multiple
OOL field accesses to the same wasm object.
This is a bit tricky because we must ensure that neither an IL-data-field nor
OOL-data-field write to the object invalidate the OOL-block-pointer read.
Hence the OOL-block-pointer-field cannot be in the same alias set as either
the IL- nor OOL-data fields. And so this patch adds three new alias-set
descriptors.
The implementation is straightforward and described in comments.
Because it is easy to mess up optimisation with incorrect alias set
descriptors, the `MWasm{Load,Store}Field*::New` methods heavily restrict
what descriptors they accept, via assertions.
Because those same MIR nodes are also used to implement exceptions, they also
accept `AliasSet::{Load,Store}(AliasSet::All)` ("no information") descriptors.
The exception-handling MIR is unaffected by this patch.
Differential Revision: https://phabricator.services.mozilla.com/D161254
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