Optimize '(X - N * M) / N' to 'X / N - M' if valid

Optimize '(X - N * M) / N' to 'X / N - M' if valid

Integer expression "(X - N * M) / N" can be optimized to "X / N - M" with
the below conditions:
1. There is no wrap/overflow/underflow.
   wrap/overflow/underflow breaks the arithmetic operation.
2. "X - N * M" and "X" are not of opposite sign.
   Here, the operation "/" would be "trunc_div", the fractional part is
   discarded. If "X - N * M" and "X" are in different signs, then trunc_div
   discards the fractional parts (of /N) in different directions.

	PR tree-optimization/108757

gcc/ChangeLog:

	* match.pd ((X - N * M) / N): New pattern.
	((X + N * M) / N): New pattern.
	((X + C) div_rshift N): New pattern.

gcc/testsuite/ChangeLog:

	* gcc.dg/pr108757-1.c: New test.
	* gcc.dg/pr108757-2.c: New test.
	* gcc.dg/pr108757.h: New test.