PR middle-end/104140: bootstrap ICE on riscv.

This patch resolves the P1 "ice-on-valid-code" regression boostrapping
GCC on risv-unknown-linux-gnu caused by my recent MULT_HIGHPART_EXPR
functionality.  RISC-V differs from x86_64 and many targets by
supporting a usmusidi3 instruction, basically a widening multiply
where one operand is signed and the other is unsigned.  Alas the
final version of my patch to recognize MULT_HIGHPART_EXPR didn't
sufficiently defend against the operands of WIDEN_MULT_EXPR having
different signedness.  This is fixed by the two-line change to
tree-ssa-math-opts.cc's convert_mult_to_highpart in the patch below.

The majority of the rest of the patch is to the documentation
(in tree.def and generic.texi).  It turns out that WIDEN_MULT_EXPR
wasn't previously documented in generic.texi, let alone the slightly
unusual semantics of allowing mismatched (signed vs unsigned) operands.
This also clarifies that MULT_HIGHPART_EXPR currently requires the
signedness of operands to match [but this might change in a future
release of GCC to support targets with usmul<mode>3_highpart].

The one final chunk of this patch (that is hopefully sufficiently
close to obvious for stage 4) is a similar (NULL pointer) sanity
check in riscv_cpu_cpp_builtins.  Currently running cc1 from the
command line (or from gdb) without specifying -march results in a
segmentation fault (ICE).  This is a minor annoyance tracking down
issues (in cross compilers) for riscv, and trivially fixed as below.

2022-01-22  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR middle-end/104140
	* tree-ssa-math-opts.cc (convert_mult_to_highpart): Check that the
	operands of the widening multiplication are either both signed or
	both unsigned, and abort the conversion if mismatched.
	* doc/generic.texi (WIDEN_MULT_EXPR): Describe expression node.
	(MULT_HIGHPART_EXPR): Clarify that operands must have the same
	signedness.
	* tree.def (MULT_HIGHPART_EXPR): Document both operands must have
	integer types with the same precision and signedness.
	(WIDEN_MULT_EXPR): Document that operands must have integer types
	with the same precision, but possibly differing signedness.
	* config/riscv/riscv-c.cc (riscv_cpu_cpp_builtins): Defend against
	riscv_current_subset_list returning a NULL pointer (empty list).

gcc/testsuite/ChangeLog
	PR middle-end/104140
	* gcc.target/riscv/pr104140.c: New test case.

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