bpf: correct zero_extend output templates

bpf: correct zero_extend output templates

The output templates for zero_extendhidi2 and zero_extendqidi2 could
lead to incorrect code generation when zero-extending one register into
another. This patch adds a new output template to the define_insns to
handle such cases and produce correct asm.

gcc/ChangeLog:
	* config/bpf/bpf.md (zero_extendhidi2): Add new output template
	for register-to-register extensions.
	(zero_extendqidi2): Likewise.