combine_simplify_rtx (doesn't) commute XOR and ASHIFTRT ???

[Alan Lawrence]

I'm looking at git commit ea1ac559 / svn r76965, January 2014 (archive:
https://gcc.gnu.org/ml/gcc-patches/2004-01/msg03406.html), which prevents

(ashiftrt (xor A C1) C2)

from being commuted to

(xor (ashiftrt A C2) (ashiftrt C1 C2))

and wondering if anyone can explain to me what's wrong with this transformation. 
Having worked through all four cases of A and C1 positive and negative, it seems 
to me that the extra bits 'fed in' to the most-significant end of the result are 
the same either way (i.e. the XOR of the sign bits of A and C1).

Re-enabling this transformation improves rtx simplification and codegen on
AArch64, for one.

(I don't have easy access to Alpha/VMS machine on which to test Ada s-crc32.adb.)

Thanks for any info, Alan