It appears that the *arm_extendqisi pattern never match anything for
reasons I can't find.  This means that on ARMv4 and ARMv5, this code:

int foo(signed char *x) { return *x; }

produces:

        ldrb    r0, [r0, #4]
        mov     r0, r0, asl #24
        mov     r0, r0, asr #24
        mov     pc, lr

instead of the expected:

        ldrsb   r0, [r0, #4]
        mov     pc, lr