Fix Issue 20136 - opEquals not recognized for AA key

[RazvanN7]

[1] Adds a v.needThis check that verifies that the variable has the STC.field storage set, however this field is set only after semantic has been ran on a var declaration.

This patch makes it so that check is valid only if semantic has been ran on that specific variable.

[1] #9884

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20136	regression	opEquals not recognized for AA key

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "stable + dmd#10320"