target/arm/translate.c: Delete/amend incorrect comments

In arm_tr_init_disas_context() we have a FIXME comment that suggests
"cpu_M0 can probably be the same as cpu_V0".  This isn't in fact
possible: cpu_V0 is used as a temporary inside gen_iwmmxt_shift(),
and that function is called in various places where cpu_M0 contains a
live value (i.e.  between gen_op_iwmmxt_movq_M0_wRn() and
gen_op_iwmmxt_movq_wRn_M0() calls).  Remove the comment.

We also have a comment on the declarations of cpu_V0/V1/M0 which
claims they're "for efficiency".  This isn't true with modern TCG, so
replace this comment with one which notes that they're only used with
the iwmmxt decode.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200803132815.3861-1-peter.maydell@linaro.org

target/arm/translate.c

@@ -57,8 +57,9 @@
 #define IS_USER(s) (s->user)
 #endif
 
-/* We reuse the same 64-bit temporaries for efficiency.  */
+/* These are TCG temporaries used only by the legacy iwMMXt decoder */
 static TCGv_i64 cpu_V0, cpu_V1, cpu_M0;
+/* These are TCG globals which alias CPUARMState fields */
 static TCGv_i32 cpu_R[16];
 TCGv_i32 cpu_CF, cpu_NF, cpu_VF, cpu_ZF;
 TCGv_i64 cpu_exclusive_addr;
@@ -8566,7 +8567,6 @@ static void arm_tr_init_disas_context(DisasContextBase *dcbase, CPUState *cs)
 
     cpu_V0 = tcg_temp_new_i64();
     cpu_V1 = tcg_temp_new_i64();
-    /* FIXME: cpu_M0 can probably be the same as cpu_V0.  */
     cpu_M0 = tcg_temp_new_i64();
 }