[PATCH] [AArch64] Support ISD::SIGN_EXTEND_INREG

[Jiangning Liu]

Hi 
Ana,

I have two reasons for handling this situation in "combine"
 stage.

1) If we customize lowering sext_inreg, we would probably missing the case
of explicitly using shl/ashr in LLVM IR. I added three more cases with
suffix "_2" to cover this kind of test. If we want to cover all in
legalization, we would have to write more codes, I think.
2) LLVM documentation says "Subsequent runs of the pass clean up the messy
code generated by the Legalize passes, which allows Legalize to be very
simple (it can focus on making code legal instead of focusing on generating
good and legal code)."

Kevin already upstreamed the improvement of generating permutation
instructions, so now I'm uploading new patches by
1) Checking uzip1 in the test
2) Adding three more cases for explicity shl/ashr pair.

Thanks,
-Jiangning
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