[llvm] [SPIRV] Legalize long vectors in GlobalISel (PR #164634)

[Steven Perron via llvm-commits]

s-perron wrote:

We could keep the bitcast intrinsic. I could add a custom rule in the legalizer to turn the intrinsic in a G_Bitcast if contains large vectors. Use the existing rules to legalize the G_Bitcast, and then turn a G_Bitcast back into the bitcast intrinsic after legalization. That might be the better way to do it. A G_Copy could get optimized away, and cause other problems.

https://github.com/llvm/llvm-project/pull/164634