When SimpleRegisterCoalescing is trimming kill flags on a physical register operand, also check if subregisters are killed.
When SimpleRegisterCoalescing is trimming kill flags on a physical register
operand, also check if subregisters are killed.

Add <imp-def> operands for subregisters that remain alive after a super register
is killed.

I don't have a testcase for this that reproduces on trunk. <rdar://problem/8441758>

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@116940 91177308-0d34-0410-b5e6-96231b3b80d8

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