[DAG] MatchLoadCombine - match swapped loads

Sort of an extension to the BSWAP pattern code:

```cpp
uint64_t ll(uint32_t *p) {
uint64_t h = *p++;
h <<= 32;
h |= *p++;
return h;
}
```
```asm
ll:
movl (%rdi), %ecx
movl 4(%rdi), %eax
shlq $32, %rcx
orq %rcx, %rax
retq
```
These i32 loads are swapped preventing them being combined into a single i64 load.

But assuming we can cheaply rotate them then we can load the i64 and then rotate it:
```cpp
uint64_t rr(uint32_t *p) {
uint64_t h = *p++;
h |= (uint64_t)*p++ << 32;
return __builtin_rotateright64(h, 32);
}
```
```asm
rr:
rorxq $32, (%rdi), %rax
retq
```
```ll
define i64 @src(ptr %p) {
%p.hi = getelementptr inbounds nuw i8, ptr %p, i64 4
%lo = load i32, ptr %p
%hi = load i32, ptr %p.hi
%conv = zext i32 %lo to i64
%shl = shl nuw i64 %conv, 32
%conv2 = zext i32 %hi to i64
%or = or disjoint i64 %shl, %conv2
ret i64 %or
}

define i64 @tgt(ptr %p) {
%p.hi = getelementptr inbounds nuw i8, ptr %p, i64 4
%lo = load i32, ptr %p
%hi = load i32, ptr %p.hi
%conv = zext i32 %hi to i64
%shl = shl nuw i64 %conv, 32
%conv2 = zext i32 %lo to i64
%or = or disjoint i64 %shl, %conv2
%rot = tail call i64 @llvm.fshl.i64(i64 %or, i64 %or, i64 32)
ret i64 %rot
}
declare i64 @llvm.fshl.i64(i64, i64, i64) #1
```