expmed: Fix store_integral_bit_field [PR101562]

expmed: Fix store_integral_bit_field [PR101562]

Our documentation says that paradoxical subregs shouldn't appear
in strict_low_part:
'(strict_low_part (subreg:M (reg:N R) 0))'
     This expression code is used in only one context: as the
     destination operand of a 'set' expression.  In addition, the
     operand of this expression must be a non-paradoxical 'subreg'
     expression.
but on the testcase below that triggers UB at runtime
store_integral_bit_field emits exactly that.

The following patch fixes it by ensuring the requirement is satisfied.

2021-07-23  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/101562
	* expmed.c (store_integral_bit_field): Only use movstrict_optab
	if the operand isn't paradoxical.

	* gcc.c-torture/compile/pr101562.c: New test.

(cherry picked from commit 8408d34570c9fe9f3d22a25a76df2a4c64f08477)