[xstormy16] Improved SImode shifts by two bits.

[xstormy16] Improved SImode shifts by two bits.

Currently on xstormy16 SImode shifts by a single bit require two
instructions, and shifts by other non-zero integer immediate constants
require five instructions.  This patch implements the obvious optimization
that shifts by two bits can be done in four instructions, by using two
single-bit sequences.

Hence, ashift_2 was previously generated as:
        mov r7,r2 | shl r2,#2 | shl r3,#2 | shr r7,#14 | or r3,r7
        ret
and with this patch we now generate:
        shl r2,#1 | rlc r3,#1 | shl r2,#1 | rlc r3,#1
        ret

2023-04-23  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/stormy16/stormy16.cc (xstormy16_output_shift): Implement
	SImode shifts by two by performing a single bit SImode shift twice.

gcc/testsuite/ChangeLog
	* gcc.target/xstormy16/shiftsi.c: New test case.