[DebugInfo] Handle dbg.assigns in FastISel (#80734)

There are some rare circumstances where dbg.assign intrinsics can reach
FastISel. They are a more specialised kind of dbg.value intrinsic with
m

[DebugInfo] Handle dbg.assigns in FastISel (#80734)

There are some rare circumstances where dbg.assign intrinsics can reach
FastISel. They are a more specialised kind of dbg.value intrinsic with
more information about the originating alloca. They only occur during
optimisation, but might reach FastISel through always_inlining an
optimised function into an optnone function.

This is a slight problem as it's not safe (for debug-info accuracy) to
ignore any intrinsics, and for RemoveDIs (the intrinsic-replacement
project) it causes a crash through an unhandled switch case. To get
around this, we can just treat the dbg.assign as a dbg.value (it's an
actual subclass) and use the variable location information from the
dbg.value fields. This loses a small amount of debug-info about stack
locations, but is more accurate than just ignoring the intrinsic.

(This has popped up deep in an LTO build of a large codebase while
testing RemoveDIs, I figured it'd be good to fix it for the
intrinsic-form at the same time, just to demonstrate the correct
behaviour).

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