[IR] Store Metadata attachments in vector (#189551)
Metadata is accessed rather often, so store metadata attachments in a
vector indexed by a number instead of a map from Value*; inside the
vector, the metadata attachments form a linked list. Instruction and
GlobalObject store the first metadata attachment index directly. Care is
taken that the offset in the structures is identical.
In addition to substantial performance improvements, this also leads to
a measurable reduction in memory usage -- several instructions have
metadata in debug builds (debug metadata) and release builds (primarily
TBAA), causing the hash table to become quite large.
In Instruction, this uses 4 padding bytes (it might be debatable whether
this is the best use for this space); GlobalObject grows by 4 bytes.
However, should we want grow Instruction by 8 bytes in the future, the
additionally used memory will be ~similar to the memory saved here, so
we could do so without causing a release-to-release memory regression.
This also makes the HasMetadata bit redundant, cleanup will follow
separately -- this is a bit annoying, because this should be inline, but
Value can't access subclass members.
