re PR rtl-optimization/87761 ([MIPS] New FAIL: gcc.target/mips/fix-r4000-10.c   -O1 start with r265398)

	PR rtl-optimization/87761
	* regcprop.c (copyprop_hardreg_forward_1): Use REG_UNUSED notes to
	detect obviously dead insns and delete them.

From-SVN: r269218