Also preserve SUBREG_PROMOTED_VAR_P in expr.c's convert_move.

Also preserve SUBREG_PROMOTED_VAR_P in expr.c's convert_move.

This patch catches another place in the middle-end where it's possible
to preserve the SUBREG_PROMOTED_VAR_P annotation on a subreg to the
benefit of later RTL optimizations.  This adds the same logic to
expr.c's convert_move as recently added to convert_modes.

On nvptx-none, the simple test program:

short foo (char c) { return c; }

currently generates three instructions:

mov.u32	%r23, %ar0;
cvt.u16.u32     %r24, %r23;
cvt.s32.s16     %value, %r24;

with this patch, we now generate just one:

mov.u32 %value, %ar0;

This patch should look familiar, it's almost identical to the recent patch
https://gcc.gnu.org/pipermail/gcc-patches/2021-August/578331.html but with
the fix https://gcc.gnu.org/pipermail/gcc-patches/2021-August/578519.html

2021-09-12  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* expr.c (convert_move): Preserve SUBREG_PROMOTED_VAR_P when
	creating a (wider) partial subreg from a SUBREG_PROMOTED_VAR_P
	subreg.