LegalizeIntegerTypes: Reenable the large shift with small amount optimization.

To avoid problems with zero shifts when getting the bits that move between words
we use a trick: first shift the by amount-1, then do another shift by one. When
amount is 0 (and size 32) we first shift by 31, then by one, instead of by 32.

Also fix a latent bug that emitted the low and high words in the wrong order
when shifting right.

Fixes PR12113.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@151637 91177308-0d34-0410-b5e6-96231b3b80d8

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