[xstormy16] Add extendhisi2 and zero_extendhisi2 patterns to stormy16.md

[xstormy16] Add extendhisi2 and zero_extendhisi2 patterns to stormy16.md

This patch adds a pair of define_insn patterns to the xstormy16 machine
description that provide extendhisi2 and zero_extendhisi2, i.e. 16-bit
to 32-bit sign- and zero-extension respectively.  This functionality is
already synthesized during RTL expansion, but providing patterns allow
the semantics to be exposed to the RTL optimizers.  To simplify things,
this patch introduces a new %h0 output format, for emitting the high_part
register name of a double-word (SImode) register pair.  The actual
code generated is identical to before.

Whilst there, I also fixed the instruction lengths and formatting of
the zero_extendqihi2 pattern.  Then, mostly for documentation purposes
as the 'T' constraint isn't yet implemented, I've added a "and Rx,#255"
alternative to zero_extendqihi2 that takes advantage of its efficient
instruction encoding.

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

gcc/ChangeLog
	* config/stormy16/stormy16.cc (xstormy16_print_operand): Add %h
	format specifier to output high_part register name of SImode reg.
	* config/stormy16/stormy16.md (extendhisi2): New define_insn.
	(zero_extendqihi2): Fix lengths, consistent formatting and add
	"and Rx,#255" alternative, for documentation purposes.
	(zero_extendhisi2): New define_insn.

gcc/testsuite/ChangeLog
	* gcc.target/xstormy16/extendhisi2.c: New test case.
	* gcc.target/xstormy16/zextendhisi2.c: Likewise.