This change sink a spill into a successor, if reloads from the stack slot
are reachable only through the successor.
For the machine IR below, we will sink the store to %stack.0 in bb.0 into
bb.1 because there is no reload from stack.0 in bb.2.
bb.0: STRXui $x0, %stack.0, 0 Bcc 11, %bb.2 bb.1: $x0 = LDRXui %stack.0, 0 RET $x0 bb.2: $x0 = COPY $xzr RET $x0