optimize sign extension

[ZERICO2005]

Here are some common patterns that can be optimized (based on usages in khicas on ez80-clang 19.1.0):

361 times:

; current
	rlc	a
	sbc	hl, hl

; optimal
	rlca
	sbc	hl, hl

97 times:

; current
	ld	l, a
	rlc	l
	sbc	hl, hl

; optimal
	rlca
	rrca ; note that this can be removed all bits in A are the same (so A = 0 or A = -1)
	sbc	hl, hl

250 times:

; current
	rlc	a
	sbc	a, a

; optimal
	rlca
	sbc	a, a

I suppose more generally, rlc a and rrc a can be transformed into rlca and rrca if only the carry flag is needed