Hexagon (target/hexagon) Use direct block chaining for tight loops

Direct block chaining is documented here
https://qemu.readthedocs.io/en/latest/devel/tcg.html#direct-block-chaining

Hexagon inner loops end with the endloop0 instruction
To go back to the beginning of the loop, this instructions writes to PC
from register SA0 (start address 0).  To use direct block chaining, we
have to assign PC with a constant value.  So, we specialize the code
generation when the start of the translation block is equal to SA0.

When this is the case, we defer the compare/branch from endloop0 to
gen_end_tb.  When this is done, we can assign the start address of the TB
to PC.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Taylor Simpson <tsimpson@quicinc.com>
Message-Id: <20221108162906.3166-12-tsimpson@quicinc.com>

target/hexagon/translate.h

@@ -59,6 +59,7 @@ typedef struct DisasContext {
     bool pre_commit;
     TCGCond branch_cond;
     target_ulong branch_dest;
+    bool is_tight_loop;
 } DisasContext;
 
 static inline void ctx_log_reg_write(DisasContext *ctx, int rnum)