[CIR] Implement atomic cmp exhange with non-const 'weak' lowering (#198546)
This was left as an NYI, but appears in self build!
This patch follows the existing solution in that we are doing the
branching of weak vs not-weak at the CIR level. This is necessary
because the LLVM intrinsics (and the CIR operaions) take 'weak' as a
constant value.
Unlike classic-codegen, this patch uses an 'if' instead of a 'switch' on
the 'weak' value. This is mainly for readability (since it is a switch
over a bool!), but also because our 'switch' doesn't seem to support
'bool', so this would require an additional cast.
As a future direction, we may wish to modify the CIR operations to take
'weak' and 'failure' value (both are constants in LLVM intrinsics!) as
non-constants, and handle the switch/if statement during lowering. This
would give us an opportunity to optimize the value out without having to
collapse the if/switch/etc, and minimize the size of the CIR. However,
as that is a larger direction, this patch skips that for now.
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