[AMDGPU][GlobalISel] Assertion failure combining shuffle_vector

[jayfoad]

After #124527 with this test case I get:

$ llc -mtriple=amdgcn -mcpu=gfx1100 -global-isel r.txt
llc: lib/CodeGen/GlobalISel/MachineIRBuilder.cpp:1400: virtual MachineInstrBuilder llvm::MachineIRBuilder::buildInstr(unsigned int, ArrayRef<DstOp>, ArrayRef<SrcOp>, std::optional<unsigned int>): Assertion `DstOps[0].getLLTTy(*getMRI()).isVector() && "Res type must be a vector"' failed.

[llvmbot]

@llvm/issue-subscribers-backend-amdgpu

Author: Jay Foad (jayfoad)

After #124527 with this [test case](https://github.com/user-attachments/files/20190734/r.txt) I get: ``` $ llc -mtriple=amdgcn -mcpu=gfx1100 -global-isel r.txt llc: lib/CodeGen/GlobalISel/MachineIRBuilder.cpp:1400: virtual MachineInstrBuilder llvm::MachineIRBuilder::buildInstr(unsigned int, ArrayRef<DstOp>, ArrayRef<SrcOp>, std::optional<unsigned int>): Assertion `DstOps[0].getLLTTy(*getMRI()).isVector() && "Res type must be a vector"' failed. ```

[lialan]

Thank you, looking into this. @jayfoad