[MachineSinking] Clear kill flag of all operands at all their uses.

When sinking an instruction it might be moved past the original last use of one
of its operands. This last use has the kill flag set and the verifier will
obviously complain about this.

Before Machine Sinking (AArch64):
%vreg3<def> = ASRVXr %vreg1, %vreg2<kill>
%XZR<def> = SUBSXrs %vreg4, %vreg1<kill>, 160, %NZCV<imp-def>
...

After Machine Sinking:
%XZR<def> = SUBSXrs %vreg4, %vreg1<kill>, 160, %NZCV<imp-def>
...
%vreg3<def> = ASRVXr %vreg1, %vreg2<kill>

This fix clears all the kill flags in all instruction that use the same operands
as the instruction that is being sunk.

This fixes rdar://problem/18180996.
[FastISel][AArch64] Use the correct register class for branches.

Also constrain the register class for branches.

This fixes rdar://problem/18181496.
[MachineCombiner][AArch64] Use the correct register class for MADD, SUB, and OR.

Select the correct register class for the various instructions that are
generated when combining instructions and constrain the registers to the
appropriate register class.

This fixes rdar://problem/18183707.