ARM: Fix PR49049

[Bernd Schmidt <bernds at codesourcery dot com>]

This corrects a brain fart in one of my patches last year: I added
another alternative to a subsi for subtraction of a constant. This is
bogus because such an operation should be canonicalized to a PLUS with
the negative constant, Normally that's what happens, and so testing
never showed that the alternative was only half-finished and didn't
work. PR49049 is a testcase where we do end up replacing a REG with a
constant and produce the bad alternative, leading to a crash.

Tested on arm-eabi and committed as obvious. Will do some sanity checks
on 4.6 and commit there as well.


Bernd

Attachment: 49049.diff
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