[ARM] Test unroll behaviour on machines with low overhead branching

Current behaviour is the single loop with fmul gets runtime unrolled by
count of 4, with the loop remainder unrolled as the 3 for.body9.us.prol
sections. This is quite a lot of compare and branch, negating the
benefits of the low overhead loop mechanism.

Author: VladiKrapp-Arm

llvm/test/Transforms/LoopUnroll/ARM/lob-unroll.ll

@@ -0,0 +1,63 @@
+; RUN: opt -mcpu=cortex-m55 -mtriple=thumbv8.1m.main -passes=loop-unroll -S  %s -o - | FileCheck %s --check-prefix=LOB
+
+; This test checks behaviour of loop unrolling on processors with low overhead branching available 
+
+; LOB-CHECK-LABEL: for.body{{.*}}.prol
+; LOB-COUNT-1:     fmul fast float 
+; LOB-CHECK-LABEL: for.body{{.*}}.prol.1
+; LOB-COUNT-1:     fmul fast float 
+; LOB-CHECK-LABEL: for.body{{.*}}.prol.2
+; LOB-COUNT-1:     fmul fast float 
+; LOB-CHECK-LABEL: for.body{{.*}}
+; LOB-COUNT-4:     fmul fast float 
+; LOB-NOT:     fmul fast float 
+
+; Function Attrs: nofree norecurse nosync nounwind memory(argmem: readwrite)
+define dso_local void @test(i32 noundef %n, ptr nocapture noundef %pA) local_unnamed_addr #0 {
+entry:
+  %cmp46 = icmp sgt i32 %n, 0
+  br i1 %cmp46, label %for.body, label %for.cond.cleanup
+
+for.cond.loopexit:                                ; preds = %for.cond6.for.cond.cleanup8_crit_edge.us, %for.body
+  %exitcond49.not = icmp eq i32 %add, %n
+  br i1 %exitcond49.not, label %for.cond.cleanup, label %for.body
+
+for.cond.cleanup:                                 ; preds = %for.cond.loopexit, %entry
+  ret void
+
+for.body:                                         ; preds = %entry, %for.cond.loopexit
+  %k.047 = phi i32 [ %add, %for.cond.loopexit ], [ 0, %entry ]
+  %add = add nuw nsw i32 %k.047, 1
+  %cmp244 = icmp slt i32 %add, %n
+  br i1 %cmp244, label %for.cond6.preheader.lr.ph, label %for.cond.loopexit
+
+for.cond6.preheader.lr.ph:                        ; preds = %for.body
+  %invariant.gep = getelementptr float, ptr %pA, i32 %k.047
+  br label %for.cond6.preheader.us
+
+for.cond6.preheader.us:                           ; preds = %for.cond6.for.cond.cleanup8_crit_edge.us, %for.cond6.preheader.lr.ph
+  %w.045.us = phi i32 [ %add, %for.cond6.preheader.lr.ph ], [ %inc19.us, %for.cond6.for.cond.cleanup8_crit_edge.us ]
+  %mul.us = mul nuw nsw i32 %w.045.us, %n
+  %0 = getelementptr float, ptr %pA, i32 %mul.us
+  %arrayidx.us = getelementptr float, ptr %0, i32 %k.047
+  br label %for.body9.us
+
+for.body9.us:                                     ; preds = %for.cond6.preheader.us, %for.body9.us
+  %x.043.us = phi i32 [ %add, %for.cond6.preheader.us ], [ %inc.us, %for.body9.us ]
+  %1 = load float, ptr %arrayidx.us, align 4
+  %mul11.us = mul nuw nsw i32 %x.043.us, %n
+  %gep.us = getelementptr float, ptr %invariant.gep, i32 %mul11.us
+  %2 = load float, ptr %gep.us, align 4
+  %mul14.us = fmul fast float %2, %1
+  %arrayidx17.us = getelementptr float, ptr %0, i32 %x.043.us
+  store float %mul14.us, ptr %arrayidx17.us, align 4
+  %inc.us = add nuw nsw i32 %x.043.us, 1
+  %exitcond.not = icmp eq i32 %inc.us, %n
+  br i1 %exitcond.not, label %for.cond6.for.cond.cleanup8_crit_edge.us, label %for.body9.us
+
+for.cond6.for.cond.cleanup8_crit_edge.us:         ; preds = %for.body9.us
+  %inc19.us = add nuw nsw i32 %w.045.us, 1
+  %exitcond48.not = icmp eq i32 %inc19.us, %n
+  br i1 %exitcond48.not, label %for.cond.loopexit, label %for.cond6.preheader.us
+}
+