PR rtl-optimization/110587: Reduce useless moves in compile-time hog.

This patch is one of a series of fixes for PR rtl-optimization/110587,
a compile-time regression with -O0, that attempts to address the underlying
cause.  As noted previously, the pathological test case pr28071.c contains
a large number of useless register-to-register moves that can produce
quadratic behaviour (in LRA).  These moves are generated during RTL
expansion in emit_group_load_1, where the middle-end attempts to simplify
the source before calling extract_bit_field.  This is reasonable if the
source is a complex expression (from before the tree-ssa optimizers), or
a SUBREG, or a hard register, but it's not particularly useful to copy
a pseudo register into a new pseudo register.  This patch eliminates that
redundancy.

The -fdump-tree-expand for pr28071.c compiled with -O0 currently contains
777K lines, with this patch it contains 717K lines, i.e. saving about 60K
lines (admittedly of debugging text output, but it makes the point).

2023-07-28  Roger Sayle  <roger@nextmovesoftware.com>
	    Richard Biener  <rguenther@suse.de>

gcc/ChangeLog
	PR middle-end/28071
	PR rtl-optimization/110587
	* expr.cc (emit_group_load_1): Simplify logic for calling
	force_reg on ORIG_SRC, to avoid making a copy if the source
	is already in a pseudo register.