re PR rtl-optimization/51447 (global register variable definition incorrectly removed as dead code)

gcc/
	PR rtl-optimization/51447
	* df-scan.c (df_get_entry_block_def_set): Add global regs to the set.
	* df-problems.c (df_lr_local_compute): Make global regs always live.
	* dce.c (deletable_insn_p): Make insns setting a global reg
	inherently necessary.

testsuite/
	PR rtl-optimization/51447
	* gcc.c-torture/execute/pr51447.c: New test.

From-SVN: r193453