util,nir: Move mask_sign_extend from opt_load_store_vectorize to util

While we're moving it, reformat a bit to make it match util_sign_extend
better.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17214>

src/util/u_math.h

@@ -579,6 +579,21 @@ util_bswap16(uint16_t n)
           (n << 8);
 }
 
+/**
+ * Mask and sign-extend a number
+ *
+ * The bit at position `width - 1` is replicated to all the higher bits.
+ * This makes no assumptions about the high bits of the value and will
+ * overwrite them with the sign bit.
+ */
+static inline int64_t
+util_mask_sign_extend(uint64_t val, unsigned width)
+{
+   assert(width > 0 && width <= 64);
+   unsigned shift = 64 - width;
+   return (int64_t)(val << shift) >> shift;
+}
+
 /**
  * Sign-extend a number
  *
@@ -588,10 +603,8 @@ util_bswap16(uint16_t n)
 static inline int64_t
 util_sign_extend(uint64_t val, unsigned width)
 {
-   assert(width > 0 && width <= 64);
    assert(width == 64 || val < (UINT64_C(1) << width));
-   unsigned shift = 64 - width;
-   return (int64_t)(val << shift) >> shift;
+   return util_mask_sign_extend(val, width);
 }
 
 static inline void*