LoopVectorize/iv-select-cmp: add tests for truncated IV

The current tests in iv-select-cmp.ll are not representative of clang
output of common real-world C programs, which are often written with i32
induction vars, as opposed to i64 induction vars. Hence, add five tests
corresponding to the following programs:

  int test(int *a, int n) {
    int rdx = 331;
    for (int i = 0; i < n; i++) {
      if (a[i] > 3)
        rdx = i;
    }
    return rdx;
  }

  int test(int *a) {
    int rdx = 331;
    for (int i = 0; i < 20000; i++) {
      if (a[i] > 3)
        rdx = i;
    }
    return rdx;
  }

  int test(int *a, long n) {
    int rdx = 331;
    for (int i = 0; i < n; i++) {
      if (a[i] > 3)
        rdx = i;
    }
    return rdx;
  }

  int test(int *a, unsigned n) {
    int rdx = 331;
    for (int i = 0; i < n; i++) {
      if (a[i] > 3)
        rdx = i;
    }
    return rdx;
  }

  int test(int *a) {
    int rdx = 331;
    for (long i = INT_MIN - 1; i < UINT_MAX; i++) {
      if (a[i] > 3)
        rdx = i;
    }
    return rdx;
  }

The first two can theoretically be vectorized without a runtime-check,
while the third and fourth cannot. The fifth cannot be vectorized, even
with a runtime-check.

This issue was found while reviewing D150851.

Differential Revision: https://reviews.llvm.org/D156124