fix return type of vmovl_high_s8

also add -flax-vector-conversions=none to the build

closes https://github.com/official-stockfish/Stockfish/pull/6587

No functional change

src/Makefile

@@ -515,7 +515,7 @@ ifeq ($(COMP),clang)
 	endif
 
 	CXXFLAGS += -pedantic -Wextra -Wshadow -Wmissing-prototypes \
-	            -Wconditional-uninitialized
+	            -Wconditional-uninitialized -flax-vector-conversions=none
 
 	ifeq ($(filter $(KERNEL),Darwin OpenBSD FreeBSD),)
 	ifeq ($(target_windows),)

src/nnue/simd.h

@@ -216,7 +216,7 @@ static constexpr std::uint32_t Mask[4] = {1, 2, 4, 8};
 
     #ifndef __aarch64__
 // Single instruction doesn't exist on 32-bit ARM
-inline int8x16_t vmovl_high_s8(int8x16_t val) { return vmovl_s8(vget_high_s8(val)); }
+inline int16x8_t vmovl_high_s8(int8x16_t val) { return vmovl_s8(vget_high_s8(val)); }
     #endif
 
 #else