[llvm] [DAGCombine] Transform `shl X, cttz(Y)` to `mul (Y & -Y), X` if cttz is unsupported (PR #85066)

[Matt Arsenault via llvm-commits]

================
@@ -0,0 +1,807 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 4
+; RUN: llc -mtriple=riscv32 -mattr=+m -verify-machineinstrs < %s \
----------------
arsenm wrote:

Don't need all the -verify-machineinstrs 

https://github.com/llvm/llvm-project/pull/85066