Hexagon (target/hexagon) Short-circuit packet register writes

In certain cases, we can avoid the overhead of writing to hex_new_value
and write directly to hex_gpr.  We add need_commit field to DisasContext
indicating if the end-of-packet commit is needed.  If it is not needed,
get_result_gpr() and get_result_gpr_pair() can return hex_gpr.

We pass the ctx->need_commit to helpers when needed.

Finally, we can early-exit from gen_reg_writes during packet commit.

There are a few instructions whose semantics write to the result before
reading all the inputs.  Therefore, the idef-parser generated code is
incompatible with short-circuit.  We tell idef-parser to skip them.

For debugging purposes, we add a cpu property to turn off short-circuit.
When the short-circuit property is false, we skip the analysis and force
the end-of-packet commit.

Here's a simple example of the TCG generated for
0x004000b4:  0x7800c020 {       R0 = #0x1 }

BEFORE:
 ---- 004000b4
 movi_i32 new_r0,$0x1
 mov_i32 r0,new_r0

AFTER:
 ---- 004000b4
 movi_i32 r0,$0x1

This patch reintroduces a use of check_for_attrib, so we remove the
G_GNUC_UNUSED added earlier in this series.

Signed-off-by: Taylor Simpson <tsimpson@quicinc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Brian Cain <bcain@quicinc.com>
Message-Id: <20230427230012.3800327-12-tsimpson@quicinc.com>

target/hexagon/hex_common.py

@@ -276,6 +276,9 @@ def need_pkt_has_multi_cof(tag):
     return "A_COF" in attribdict[tag]
 
 
+def need_pkt_need_commit(tag):
+    return 'A_IMPLICIT_WRITES_USR' in attribdict[tag]
+
 def need_condexec_reg(tag, regs):
     if "A_CONDEXEC" in attribdict[tag]:
         for regtype, regid, toss, numregs in regs: