py/asmarm: Implement the full set of Viper load/store operations.

This commit expands the implementation of Viper load/store operations
that are optimised for the Arm platform.

Now both load and store emitters should generate the shortest possible
sequence in all cases.  Redundant specialised operation emitters have
been folded into the general case implementation - this was the case of
integer-indexed load/store operations with a fixed offset of zero.

Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

py/emitnative.c

@@ -1785,10 +1785,6 @@ static void emit_native_store_subscr(emit_t *emit) {
                     if (index_value != 0) {
                         // index is non-zero
                         ASM_MOV_REG_IMM(emit->as, reg_index, index_value);
-                        #if N_ARM
-                        asm_arm_strb_reg_reg_reg(emit->as, reg_value, reg_base, reg_index);
-                        break;
-                        #endif
                         ASM_ADD_REG_REG(emit->as, reg_index, reg_base); // add index to base
                         reg_base = reg_index;
                     }