PR119482: Avoid mispredictions in bitmap_set_bit

PR119482: Avoid mispredictions in bitmap_set_bit

bitmap_set_bit checks the original value of the bit to return it to the
caller and then only writes the new value back if it changes.
Most callers of bitmap_set_bit don't need the return value, but with the conditional store
the CPU still has to predict it correctly since gcc doesn't know how to do
that without APX on x86 (even though CMOV could do it with a dummy target).
Really if-conversion should handle this case, but for now we can fix
it.

This simple patch improves runtime by 15% for the test case in the PR.
Which is more than I expected given it only has ~1.44% of the cycles, but I guess
the mispredicts caused some down stream effects.

  cc1plus-bitmap -std=gnu++20 -O2 pr119482.cc  -quiet
  ran 1.15 ± 0.01 times faster than cc1plus -std=gnu++20 -O2 pr119482.cc  -quiet

At least with this test case the total number of branches decreases
drastically. Even though the mispredict rate goes up slightly it is
still a big win.

$ perf stat -e branches,branch-misses,uncore_imc/cas_count_read/,uncore_imc/cas_count_write/ \
-a ../obj-fast/gcc/cc1plus -std=gnu++20 -O2 pr119482.cc  -quiet -w

 Performance counter stats for 'system wide':

    41,932,957,091      branches
       686,117,623      branch-misses                    #    1.64% of all branches
         43,690.47 MiB  uncore_imc/cas_count_read/
         12,362.56 MiB  uncore_imc/cas_count_write/

      49.328633365 seconds time elapsed

$ perf stat -e branches,branch-misses,uncore_imc/cas_count_read/,uncore_imc/cas_count_write/ \
-a ../obj-fast/gcc/cc1plus-bitmap -std=gnu++20 -O2 pr119482.cc  -quiet -w

 Performance counter stats for 'system wide':

    37,092,113,179      branches
       663,641,708      branch-misses                    #    1.79% of all branches
         43,196.52 MiB  uncore_imc/cas_count_read/
         12,369.33 MiB  uncore_imc/cas_count_write/

      42.632458350 seconds time elapsed

gcc/ChangeLog:

	PR middle-end/119482
	* bitmap.cc (bitmap_set_bit): Write back value unconditionally