[ARM] Fix crash in chained BFI combine due to incorrectly RAUW'ing a node.

For a bfi chain like:
a = bfi input, x, y
b = bfi a, x', y'

The previous code was RAUW'ing a with x, mutating the second 'b' bfi, and when
SelectionDAG's CSE code ended up deleting it unexpectedly, bad things happend.
There's no need to RAUW in this case because we can just return our newly
created replacement BFI node. It also looked incorrect because it didn't account
for other users of the 'a' bfi.

Since it seems that chains of more than 2 BFI nodes are hard/impossible to
produce without this combine kicking in at some point, I've removed that
functionality since it had no test coverage.

rdar://79095399

Differential Revision: https://reviews.llvm.org/D104868

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