[CodeGenPrepare][test] Add test for sinking of truncs demonstrating nsw/nuw are dropped

SinkCast creates a new cast with the same type and inputs, which drops
the nsw/nuw flags.

Reviewed as part of <#89904>
but split out so I can land the test separately.

Author: asb

llvm/test/Transforms/CodeGenPrepare/RISCV/noop-copy-sink.ll

@@ -0,0 +1,31 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S -passes='require<profile-summary>,function(codegenprepare)' -mtriple=riscv64 %s \
+; RUN:   | FileCheck --check-prefixes=CHECK %s
+
+define i16 @sink_trunc1(i64 %a) {
+; CHECK-LABEL: @sink_trunc1(
+; CHECK-NEXT:  fnend:
+; CHECK-NEXT:    [[TMP0:%.*]] = trunc i64 [[A:%.*]] to i16
+; CHECK-NEXT:    ret i16 [[TMP0]]
+;
+  %trunc = trunc i64 %a to i16
+  br label %fnend
+
+fnend:
+  ret i16 %trunc
+}
+
+; The flags on the original trunc should be preserved.
+; FIXME: Flags are currently dropped.
+define i16 @sink_trunc2(i64 %a) {
+; CHECK-LABEL: @sink_trunc2(
+; CHECK-NEXT:  fnend:
+; CHECK-NEXT:    [[TMP0:%.*]] = trunc i64 [[A:%.*]] to i16
+; CHECK-NEXT:    ret i16 [[TMP0]]
+;
+  %trunc = trunc nuw nsw i64 %a to i16
+  br label %fnend
+
+fnend:
+  ret i16 %trunc
+}