Add test cases showing missed opportunity to use slli.uw or shXadd.uw. NFC

[craig.topper]

Summary

The tests here show the codegen for something like this C code.

unsigned diff = ptr1 - ptr2;
return ptr3[diff];

The pointer difference is truncated to 32-bits before being used
again as an index. In SelectionDAG this appears as an AND between
a SRL and a SHL. DAGCombiner will remove the shifts leaving only
an AND. The Mask now has 1,2, or 3 trailing zeros and 31, 30, or 29
leading zeros. We end up falling back to constant materialization
to create this mask.

We could instead use srli followed by slli.uw. Or since
we have an add, we can use srli followed by shXadd.uw.

llvm/test/CodeGen/RISCV/rv64zba.ll

 ; RV64ZBAZBB-NEXT:    sext.h a0, a0
 ; RV64ZBAZBB-NEXT:    slli a0, a0, 23
 ; RV64ZBAZBB-NEXT:    srli a0, a0, 32
 ; RV64ZBAZBB-NEXT:    ret
   %ext = sext i16 %a to i32
   %1 = ashr i32 %ext, 9
   ret i32 %1
 }
+
+; This the IR you get from InstCombine if take the difference of 2 pointers and
+; cast is to unsigned before using as an index.
+define signext i16 @sh1adduw_ptrdiff(i64 %diff, i16* %baseptr) {
+; CHECK-LABEL: sh1adduw_ptrdiff:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    li a2, 1
+; CHECK-NEXT:    slli a2, a2, 33
+; CHECK-NEXT:    addi a2, a2, -2
+; CHECK-NEXT:    and a0, a0, a2
+; CHECK-NEXT:    add a0, a1, a0
+; CHECK-NEXT:    lh a0, 0(a0)
+; CHECK-NEXT:    ret
+  %ptrdiff = lshr exact i64 %diff, 1
+  %cast = and i64 %ptrdiff, 4294967295
+  %ptr = getelementptr inbounds i16, i16* %baseptr, i64 %cast
+  %res = load i16, i16* %ptr
+  ret i16 %res
+}
+
+define signext i32 @sh2adduw_ptrdiff(i64 %diff, i32* %baseptr) {
+; CHECK-LABEL: sh2adduw_ptrdiff:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    li a2, 1
+; CHECK-NEXT:    slli a2, a2, 34
+; CHECK-NEXT:    addi a2, a2, -4
+; CHECK-NEXT:    and a0, a0, a2
+; CHECK-NEXT:    add a0, a1, a0
+; CHECK-NEXT:    lw a0, 0(a0)
+; CHECK-NEXT:    ret
+  %ptrdiff = lshr exact i64 %diff, 2
+  %cast = and i64 %ptrdiff, 4294967295
+  %ptr = getelementptr inbounds i32, i32* %baseptr, i64 %cast
+  %res = load i32, i32* %ptr
+  ret i32 %res
+}
+
+define i64 @sh3adduw_ptrdiff(i64 %diff, i64* %baseptr) {
+; CHECK-LABEL: sh3adduw_ptrdiff:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    li a2, 1
+; CHECK-NEXT:    slli a2, a2, 35
+; CHECK-NEXT:    addi a2, a2, -8
+; CHECK-NEXT:    and a0, a0, a2
+; CHECK-NEXT:    add a0, a1, a0
+; CHECK-NEXT:    ld a0, 0(a0)
+; CHECK-NEXT:    ret
+  %ptrdiff = lshr exact i64 %diff, 3
+  %cast = and i64 %ptrdiff, 4294967295
+  %ptr = getelementptr inbounds i64, i64* %baseptr, i64 %cast
+  %res = load i64, i64* %ptr
+  ret i64 %res
+}