x86: Fix up ix86_atomic_assign_expand_fenv [PR94780]

x86: Fix up ix86_atomic_assign_expand_fenv [PR94780]

This function, because it is sometimes called even outside of function
bodies, uses create_tmp_var_raw rather than create_tmp_var.  But in order
for that to work, when first referenced, the VAR_DECLs need to appear in a
TARGET_EXPR so that during gimplification the var gets the right
DECL_CONTEXT and is added to local decls.  Without that, e.g. tree-nested.c
ICEs on those.

2020-04-27  Jakub Jelinek  <jakub@redhat.com>

	PR target/94780
	* config/i386/i386.c (ix86_atomic_assign_expand_fenv): Use
	TARGET_EXPR instead of MODIFY_EXPR for first assignment to
	sw_var, exceptions_var, mxcsr_orig_var and mxcsr_mod_var.

	* gcc.dg/pr94780.c: New test.

(cherry picked from commit 9b8e9006bb35641865358e2df4f6b3ae185b239a)