Clear kill flags in tail duplication.

If we duplicate an instruction then we must also clear kill flags on any uses we rewrite.
Otherwise we might be killing a register which was used in other BBs.

For example, here the entry BB ended up with these instructions, the ADD having been tail duplicated.

	%vreg24<def> = t2ADDri %vreg10<kill>, 1, pred:14, pred:%noreg, opt:%noreg; GPRnopc:%vreg24 rGPR:%vreg10
	%vreg22<def> = COPY %vreg10; GPR:%vreg22 rGPR:%vreg10

	The copy here is inserted after the add and so needs vreg10 to be live.

llvm-svn: 236782