array memset testing

[brody2consult]

NOTE: I would be happy to contribute the testing, likely based on the test cases from #586 if that is merged.

---

I discovered that the following codegen mutations do not trigger any compiletest or difftest failures:

diff --git a/crates/rustc_codegen_spirv/src/builder/builder_methods.rs b/crates/rustc_codegen_spirv/src/builder/builder_methods.rs
index d86db1cbd0..b6a867933f 100644
--- a/crates/rustc_codegen_spirv/src/builder/builder_methods.rs
+++ b/crates/rustc_codegen_spirv/src/builder/builder_methods.rs
@@ -290,21 +290,16 @@ macro_rules! simple_uni_op {
 }
 
 fn memset_fill_u16(b: u8) -> u16 {
-    b as u16 | ((b as u16) << 8)
+    b as u16
 }
 
 fn memset_fill_u32(b: u8) -> u32 {
-    b as u32 | ((b as u32) << 8) | ((b as u32) << 16) | ((b as u32) << 24)
+    b as u32
 }
 
 fn memset_fill_u64(b: u8) -> u64 {
     b as u64
         | ((b as u64) << 8)
-        | ((b as u64) << 16)
-        | ((b as u64) << 24)
-        | ((b as u64) << 32)
-        | ((b as u64) << 40)
-        | ((b as u64) << 48)
         | ((b as u64) << 56)
 }
 
@@ -384,9 +379,6 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 32 => self
                     .constant_u32(self.span(), memset_fill_u32(fill_byte))
                     .def(self),
-                64 => self
-                    .constant_u64(self.span(), memset_fill_u64(fill_byte))
-                    .def(self),
                 _ => self.fatal(format!(
                     "memset on integer width {width} not implemented yet"
                 )),

I discovered this while investigating possible testing for a more general solution to #594.

I think the test cases from PR #586 may be able to help avoid this issue if we can adapt these to test memset array fill for u64, i64, u32, etc & check the disassembly. This would give me some more confidence in case we would ever want to refactor some of this memset-related code (someday).