Add test for infinite combine fix in the previous commit

This was somewhat awkward to reduce since giving the --print-before-all
IR to llc would optimize away the pattern that triggers the loop.
Writing the test with -start-before=riscv-isel allowed me to reduce it
with bugpoint and then manually remove all branches to get the minimal
test from this comit

Author: arichardson

llvm/test/CodeGen/RISCV/cheri/ptradd-infinite-combine.ll

@@ -0,0 +1,24 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 2
+; RUN: %riscv64_cheri_purecap_llc -start-before=riscv-isel < %s | FileCheck %s
+;; This is a reduced testcase for an infinite combine that was fixed by
+;; "[CodeGen] Don't create combine loops for PTRADD with opaque constants"
+;; (commit 511c1e0089b4112d4dfbc8b3b093c588aeedb8e9)
+
+define ptr addrspace(200) @wwv_rf(ptr addrspace(200) %arg) addrspace(200) {
+; CHECK-LABEL: wwv_rf:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    lui a1, 1
+; CHECK-NEXT:    addi a1, a1, -656
+; CHECK-NEXT:    addi a1, a1, 112
+; CHECK-NEXT:    cincoffset a0, a0, a1
+; CHECK-NEXT:    cret
+entry:
+  ; We need an opaque constant here to trigger the infinite loop that keeps
+  ; changing between (ptradd arg, (add 122, opaque 3440)) and
+  ; (ptradd (ptradd arg, 122), opaque 3440). Without the opaque constant,
+  ; the addition would just be folded to 3522.
+  %const5 = bitcast i64 3440 to i64
+  %const_mat = add i64 %const5, 112
+  %scevgep2 = getelementptr i8, ptr addrspace(200) %arg, i64 %const_mat
+  ret ptr addrspace(200) %scevgep2
+}