regrename: Fix -fcompare-debug issue in check_new_reg_p [PR105041]

regrename: Fix -fcompare-debug issue in check_new_reg_p [PR105041]

In check_new_reg_p, the nregs of a du chain is computed by obtaining the
MODE of the first element in the chain, and then calling
hard_regno_nregs() with the MODE. But the first element of the chain can
be a DEBUG_INSN whose mode need not be the same as the rest of the
elements in the du chain. This was resulting in fcompare-debug failure
as check_new_reg_p was returning a different result with -g for the same
candidate register. We can instead obtain nregs from the du chain
itself.

2022-06-10  Surya Kumari Jangala  <jskumari@linux.ibm.com>

gcc/
	PR rtl-optimization/105041
	* regrename.cc (check_new_reg_p): Use nregs value from du chain.

gcc/testsuite/
	PR rtl-optimization/105041
	* gcc.target/powerpc/pr105041.c: New test.