middle-end/100775 - updating the reg use in exit block for -fzero-call-used-regs

middle-end/100775 - updating the reg use in exit block for -fzero-call-used-regs

GCC11 backport.

In the pass_zero_call_used_regs, when updating dataflow info after adding
the register zeroing sequence in the epilogue of the function, we should
call "df_update_exit_block_uses" to update the register use information in
the exit block to include all the registers that have been zeroed.

gcc/ChangeLog:

	PR middle-end/100775
	* function.c (gen_call_used_regs_seq): Call
	df_update_exit_block_uses when updating df.

gcc/testsuite/ChangeLog:

	PR middle-end/100775
	* gcc.target/arm/pr100775.c: New test.