[xla:cpu] NFC: Give human readable names to LLVM IR instructions in host kernel prototype
[xla:cpu] NFC: Give human readable names to LLVM IR instructions in host kernel prototype

Example:

```
  %tdim_x_gep = getelementptr inbounds %SE_HOST_KernelThreadDim, ptr %2, i32 0
  %tdim_y_gep = getelementptr inbounds %SE_HOST_KernelThreadDim, ptr %2, i32 1
  %tdim_z_gep = getelementptr inbounds %SE_HOST_KernelThreadDim, ptr %2, i32 2
  %tdim_x = load i64, ptr %tdim_x_gep, align 4
  %tdim_y = load i64, ptr %tdim_y_gep, align 4
  %tdim_z = load i64, ptr %tdim_z_gep, align 4
  %3 = getelementptr inbounds %SE_HOST_KernelCallFrame, ptr %0, i32 0, i32 1
  %tid_x_gep = getelementptr inbounds %SE_HOST_KernelThread, ptr %3, i32 0, i32 0
  %tid_y_gep = getelementptr inbounds %SE_HOST_KernelThread, ptr %3, i32 0, i32 1
  %tid_z_gep = getelementptr inbounds %SE_HOST_KernelThread, ptr %3, i32 0, i32 2
  %tid_x = load i64, ptr %tid_x_gep, align 4
  %tid_y = load i64, ptr %tid_y_gep, align 4
  %tid_z = load i64, ptr %tid_z_gep, align 4
  %args_gep = getelementptr inbounds %SE_HOST_KernelCallFrame, ptr %0
  %args = load ptr, ptr %args_gep, align 8
  %arg0_gep = getelementptr %SE_HOST_KernelArg, ptr %args, i32 0, i32 0
  %arg0 = load ptr, ptr %arg0_gep, align 8
  %args_gep1 = getelementptr inbounds %SE_HOST_KernelCallFrame, ptr %0
  %args2 = load ptr, ptr %args_gep1, align 8
  %arg1_gep = getelementptr %SE_HOST_KernelArg, ptr %args2, i32 1, i32 0
  %arg1 = load ptr, ptr %arg1_gep, align 8
```
