[SPIR-V] Unimplemented implicit down-casts from bitcasted loads

[winocm]

This is a minimal test case that demonstrates the issue:

; RUN: llc -verify-machineinstrs -O0 -mtriple=spirv-vulkan1.3-unknown %s -o - | FileCheck %s

@bar = external addrspace(2) externally_initialized global i32

define void @foo() {
  %load = load <4 x i8>, ptr addrspace(2) @bar, align 4
  ret void
}

Loads (and probably stores) that are effectively the same size as the target variable require intermediate bitcasts.