Fix SPIRV validation: use array ptr for cooperative vector ops

[jkwak-work]

Summary

• OpCooperativeVectorMatrixMulNV (and related NV cooperative vector ops) require the matrix/buffer operand to be a pointer to an array type, not a pointer to a struct
• After SPIRV legalization, ByteAddressBuffer/StructuredBuffer global params become ptr-to-struct{runtimeArray<T>}, which fails the SPIRV validator with: Pointer <id>'s Type must be an array type
• Adds emitBufferPtrAsArrayPtr() helper that emits OpAccessChain index 0 to pierce through the wrapper struct when needed; for Ptr<T[]> inputs that already point directly to an unsized array, the pointer is returned as-is

Affected functions

• emitCoopVecMatMulAdd — matrix ptr and bias ptr
• emitCoopVecOuterProductAccumulate — matrix ptr
• emitCoopVecReduceSumAccumulate — buffer ptr

Test

SLANG_RUN_SPIRV_VALIDATION=1 build/Debug/bin/slangc.exe \
  -entry computeMain -stage compute -target spirv-asm \
  tests/cooperative-vector/matrix-mul.slang

Before: error: line 80: spv::Op::OpCooperativeVectorMatrixMulNV Pointer <id>'s Type must be an array type.
After: compiles cleanly, generates correct OpAccessChain before the matrix mul op.

All 214 cooperative-vector tests pass.