SelectionDAG: fix logic for promoting shift types
SelectionDAG: fix logic for promoting shift types

r238503 fixed the problem of too-small shift types by promoting them
during legalization, but the correct solution is to promote only the
operands that actually demand promotion.

This fixes a crash on an out-of-tree target caused by trying to
promote an operand that can't be promoted.


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@238632 91177308-0d34-0410-b5e6-96231b3b80d8

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