[ARM] Fix VMOVRRD combine with non-canonical inserts. (#109639)
In some situations, in the test case here with the multiple calls being
late legalized, we can see inserts of the form:
```
b = insert a, x, 0
c = insert b, y, 1
d = insert c, z, 0
bc = bitcast d
e = extract bc, 0
r = vmovrrd e
```
The redundant insert will usually be removed, but in some cases are not
prior to PerformVMOVRRDCombine. The code was finding the first insert
from each lane (x and y), as opposed to the last (z and y).
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