Improved SUBREG simplifications in simplify-rtx.cc's simplify_subreg.

Improved SUBREG simplifications in simplify-rtx.cc's simplify_subreg.

An x86 backend improvement that I'm working results in combine attempting
to recognize:

(set (reg:DI 87 [ xD.2846 ])
     (ior:DI (subreg:DI (ashift:TI (zero_extend:TI (reg:DI 92))
                                   (const_int 64 [0x40])) 0)
             (reg:DI 91)))

where the lowpart SUBREG has difficulty seeing through the (hi<<64)
that the lowpart must be zero.  Rather than workaround this in the
backend, the better fix is to teach simplify-rtx that
lowpart((hi<<64)|lo) -> lo and highpart((hi<<64)|lo) -> hi, so that
all backends benefit.  Reducing the number of places where the
middle-end generates a SUBREG of something other than REG is a
good thing.

On x86_64-pc-linux-gnu, the testcase pr78904-1b.c FAILs with this patch,
due to changes in expected/canonical RTL, for which a backend patch to
i386.md has already been provisionally approved.

2023-06-23  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* simplify-rtx.cc (simplify_subreg):  Optimize lowpart SUBREGs
	of ASHIFT to const0_rtx with sufficiently large shift count.
	Optimize highpart SUBREGs of ASHIFT as the shift operand when
	the shift count is the correct offset.  Optimize SUBREGs of
	multi-word logic operations if the SUBREGs of both operands
	can be simplified.