a couple of small patches for simd.h

0001 essentially reverts commit c6a43c2, and instead fixes the problem
(MSVC dislikes casts to the same type) by omitting the cast in the
problematic line in pg_lfind32().  While working on optimizing hex_encode()
and hex_decode() [0], I noticed that implicit conversions sufficed.

0002 optimizes vector8_has_le() on AArch64 by using vminvq_u8().  I needed
vector8_has_ge() for the hex_encode()/hex_decode() work and noticed this
opportunity.

[0] https://commitfest.postgresql.org/patch/5538/

-- 
nathan