grep: use grep_and_expr() in compile_pattern_and()

grep: use grep_and_expr() in compile_pattern_and()

In a similar spirit as a previous commit, use the new `grep_and_expr()`
to construct the AND node in `compile_pattern_and()`.

Unlike the aforementioned previous commit, this is not about code
duplication, since this is the only spot in grep.c where an AND node is
constructed. Rather, this is about visual consistency with the other
`compile_pattern_xyz()` functions.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>