[LoopUnroll] Add multi-exit test which does not exit through latch.

This patch adds a new test for loop-unrolling with multiple exiting
blocks, where the latch does not exit, but the header does. This can
happen when the loop has not been rotated, e.g. due to minsize.

Inspired by the following end-to-end test, using -Oz
https://godbolt.org/z/fP6sna8qK

    bool foo(int *ptr, int limit) {
        #pragma clang loop unroll(full)
        for (unsigned int i = 0; i < 4; i++) {
            if (ptr[i] > limit)
            return false;
            ptr[i]++;
        }
        return true;
    }

Author: fhahn

llvm/test/Transforms/LoopUnroll/unroll-header-exiting-with-phis-multiple-exiting-blocks.ll

@@ -0,0 +1,58 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -loop-unroll -S %s | FileCheck %s
+
+; Loop with multiple exiting blocks, where the header exits but not the latch,
+; e.g. because it has not been rotated.
+define i16 @full_unroll_multiple_exiting_blocks(i16* %A, i16 %x, i16 %y) {
+; CHECK-LABEL: @full_unroll_multiple_exiting_blocks(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[HEADER:%.*]]
+; CHECK:       header:
+; CHECK-NEXT:    [[RES:%.*]] = phi i16 [ 123, [[ENTRY:%.*]] ], [ [[RES_NEXT:%.*]], [[LATCH:%.*]] ]
+; CHECK-NEXT:    [[I_0:%.*]] = phi i64 [ 0, [[ENTRY]] ], [ [[INC9:%.*]], [[LATCH]] ]
+; CHECK-NEXT:    [[PTR:%.*]] = getelementptr inbounds i16, i16* [[A:%.*]], i64 [[I_0]]
+; CHECK-NEXT:    [[LV:%.*]] = load i16, i16* [[PTR]], align 2
+; CHECK-NEXT:    [[RES_NEXT]] = add i16 [[RES]], [[LV]]
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ult i64 [[I_0]], 3
+; CHECK-NEXT:    br i1 [[CMP]], label [[EXITING_1:%.*]], label [[EXIT:%.*]]
+; CHECK:       exiting.1:
+; CHECK-NEXT:    [[EC_1:%.*]] = icmp eq i16 [[LV]], [[X:%.*]]
+; CHECK-NEXT:    br i1 [[EC_1]], label [[EXIT]], label [[EXITING_2:%.*]]
+; CHECK:       exiting.2:
+; CHECK-NEXT:    [[EC_2:%.*]] = icmp eq i16 [[LV]], [[Y:%.*]]
+; CHECK-NEXT:    br i1 [[EC_2]], label [[EXIT]], label [[LATCH]]
+; CHECK:       latch:
+; CHECK-NEXT:    [[INC9]] = add i64 [[I_0]], 1
+; CHECK-NEXT:    br label [[HEADER]]
+; CHECK:       exit:
+; CHECK-NEXT:    [[RES_LCSSA:%.*]] = phi i16 [ [[RES_NEXT]], [[HEADER]] ], [ 0, [[EXITING_1]] ], [ 1, [[EXITING_2]] ]
+; CHECK-NEXT:    ret i16 [[RES_LCSSA]]
+;
+entry:
+  br label %header
+
+header:
+  %res = phi i16 [ 123, %entry ], [ %res.next, %latch ]
+  %i.0 = phi i64 [ 0, %entry ], [ %inc9, %latch ]
+  %ptr = getelementptr inbounds i16, i16* %A, i64 %i.0
+  %lv = load i16, i16* %ptr
+  %res.next = add i16 %res, %lv
+  %cmp = icmp ult i64 %i.0, 3
+  br i1 %cmp, label %exiting.1, label %exit
+
+exiting.1:
+  %ec.1 = icmp eq i16 %lv, %x
+  br i1 %ec.1, label %exit, label %exiting.2
+
+exiting.2:
+  %ec.2 = icmp eq i16 %lv, %y
+  br i1 %ec.2, label %exit, label %latch
+
+latch:
+  %inc9 = add i64 %i.0, 1
+  br label %header
+
+exit:
+  %res.lcssa = phi i16 [ %res.next, %header ], [ 0, %exiting.1 ], [ 1, %exiting.2 ]
+  ret i16 %res.lcssa
+}