dev-util/intel-graphics-compiler: pass $LLVM_MAX_SLOT to get_llvm_prefix()

dev-util/intel-graphics-compiler: pass $LLVM_MAX_SLOT to get_llvm_prefix()

When invoked without max_slot, get_llvm_prefix() iterates over *all*
LLVM slots known to llvm.eclass - including those exceeding LLVM_MAX_SLOT.
As a consequence, an ebuild can e.g. end up getting installed into llvm:9
directories in spite of having been linked against llvm:8.

Package-Manager: Portage-2.3.76, Repoman-2.3.16
Signed-off-by: Marek Szuba <marecki@gentoo.org>