expr: Workaround profiledbootstrap uninit false positive [PR103899]

expr: Workaround profiledbootstrap uninit false positive [PR103899]

The threader changes resulted in a false positive warning during
profiledbootstrap:
In file included from ../../gcc/expr.c:26:
../../gcc/tree.h: In function ‘rtx_def* expand_expr_real_1(tree, rtx, machine_mode, expand_modifier, rtx_def**, bool)’:
../../gcc/tree.h:244:56: error: ‘context’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
  244 | #define TREE_CODE(NODE) ((enum tree_code) (NODE)->base.code)
      |                                                        ^~~~
../../gcc/expr.c:10343:8: note: ‘context’ was declared here
10343 |   tree context;
      |        ^~~~~~~
While it will be nice to improve the uninit pass to handle it if possible
(I do not want to close the PR until that is done), doing profiledbootstrap
is a common thing to do, so a workaround is handy, especially as in this
case when the workaround seems to be the right thing to do, as it moves
a variable declaration to the only place where it is set and used and avoids
the weird and for uninit asking
  tree context;
...
  if (exp)
    context = ...;
  gcc_assert (!exp
              || use (context)
              || use_some_more (context));

2022-01-06  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/103899
	* expr.c (expand_expr_real_1): Add a workaround for bogus uninit
	warning by moving context variable to the only spot where it is used
	and moving gcc_assert into if body.