[PATCH v1] Widening-Mul: Fix one ICE of gcall insertion for PHI match

[PATCH v1] Widening-Mul: Fix one ICE of gcall insertion for PHI match

When enabled the PHI handing for COND_EXPR,  we need to insert the gcall
to replace the PHI node.  Unfortunately,  I made a mistake that insert
the gcall to before the last stmt of the bb.  See below gimple,  the PHI
is located at no.1 but we insert the gcall (aka no.9) to the end of
the bb.  Then the use of _9 in no.2 will have no def and will trigger
ICE when verify_ssa.

  1. # _9 = PHI <_3(4), 18446744073709551615(3)> // The PHI node to be deleted.
  2. prephitmp_36 = (char *) _9;
  3. buf.write_base = string_13(D);
  4. buf.write_ptr = string_13(D);
  5. buf.write_end = prephitmp_36;
  6. buf.written = 0;
  7. buf.mode = 3;
  8. _7 = buf.write_end;
  9. _9 = .SAT_ADD (string.0_2, maxlen_15(D));   // Insert gcall to last bb by mistake

This patch would like to insert the gcall to before the start of the bb
stmt.  To ensure the possible use of PHI_result will have a def exists.
After this patch the above gimple will be:

  0. _9 = .SAT_ADD (string.0_2, maxlen_15(D));   // Insert gcall to start bb by mistake
  1. # _9 = PHI <_3(4), 18446744073709551615(3)> // The PHI node to be deleted.
  2. prephitmp_36 = (char *) _9;
  3. buf.write_base = string_13(D);
  4. buf.write_ptr = string_13(D);
  5. buf.write_end = prephitmp_36;
  6. buf.written = 0;
  7. buf.mode = 3;
  8. _7 = buf.write_end;

The below test suites are passed for this patch:
* The rv64gcv fully regression test with newlib.
* The rv64gcv build with glibc.
* The x86 regression test with newlib.
* The x86 bootstrap test with newlib.

	PR target/115387

gcc/ChangeLog:

	* tree-ssa-math-opts.cc (math_opts_dom_walker::after_dom_children): Take
	the gsi of start_bb instead of last_bb.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/pr115387-1.c: New test.
	* gcc.target/riscv/pr115387-2.c: New test.