RISC-V: Fix Zicond ICE on large constants

Large constant cons and/or alt will trigger ICEs building GCC target
libraries (libgomp and libatomic) when the 'Zicond' extension is enabled.

For instance, zicond-ice-2.c (new test case in this commit) will cause
an ICE when SOME_NUMBER is 0x1000 or larger.  While opposite numbers
corresponding cons/alt (two temp2 variables) are checked, cons/alt
themselves are not checked and causing 2 ICEs building
GCC target libraries as of this writing:

1.  gcc/libatomic/config/posix/lock.c
2.  gcc/libgomp/fortran.c

Coercing a large value into a register will fix the issue.

It also coerce a large cons into a register on "imm, imm" case (the author
could not reproduce but possible to cause an ICE).

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_expand_conditional_move): Force
	large constant cons/alt into a register.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/zicond-ice-2.c: New test.  This is based on
	an ICE at libat_lock_n func on gcc/libatomic/config/posix/lock.c
	but heavily minimized.

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