The value numbering function vn() used only the 32-bit FNV hash to
determine if two nodes are equivalent for common subexpression
elimination. When two different nodes had a hash collision, the second
node was incorrectly treated as a duplicate of the first, causing it
to share the first node's register and interval. This led to the
wrong value being read at execution time.

For example, a slice op producing shape (1,1,5) could hash-collide
with a MatMul producing shape (10000,1). The MatMul would be treated
as equivalent to the slice op, and any consumer of the MatMul would
read the slice op's value instead.

The fix adds a nodeEq check to verify actual equivalence before
declaring a CSE match, consistent with how ExprGraph.AddNode already
handles hash collisions.