Clear kill flags in ARMLoadStoreOptimizer.

Clear kill flags in ARMLoadStoreOptimizer.

The pass here was clearing kill flags on instructions which had
their sources killed in the instruction being combined.  But
given that the new instruction is inserted after the existing ones,
any existing instructions with kill flags will lead to the verifier
complaining that we are reading an undefined physreg.

For example, what we had prior to this optimization is
	t2STRi12 %R1, %SP, 12
	t2STRi12 %R1<kill>, %SP, 16
	t2STRi12 %R0<kill>, %SP, 8

and prior to this fix that would generate
	t2STRi12 %R1<kill>, %SP, 16
	t2STRDi8 %R0<kill>, %R1, %SP, 8

This is clearly incorrect as it didn't clear the kill flag on R1
used with offset 16 because there was no kill flag on the instruction
with offset 12.

After this change we clear the kill flag on the offset 16 instruction
because we know it will be used afterwards in the new instruction.

I haven't provided a test case.  I have a small test, but even it is
very sensitive to register allocation order which isn't ideal.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242359 91177308-0d34-0410-b5e6-96231b3b80d8