tree-opt: Fix segfault in tree-if-conv.c with -march=armv8.2-a+sve [PR97849]

tree-opt: Fix segfault in tree-if-conv.c with -march=armv8.2-a+sve [PR97849]

The issue here is that rpo vn may eliminate target ssa_name referred to in
redundant_ssa_names, and thus ifcvt_local_dce may replace candidate
ssa_name with invalid ssa_name resulting in incorrect IR. The patch simply
does ssa_name replacement before calling do_rpo_vn, which fixes the issue.

gcc/
2020-11-24  Prathamesh Kulkarni  <prathamesh.kulkarni@linaro.org>

	PR tree-optimization/97849
	* tree-if-conv.c (tree_if_conversion): Move ssa_name
	replacement code from ifcvt_local_dce to this function
	before calling do_rpo_vn.

gcc/testsuite/
2020-11-24  Prathamesh Kulkarni  <prathamesh.kulkarni@linaro.org>

	PR tree-optimization/97849
	* gcc.dg/tree-ssa/pr97849.c: New test.