Fix min/max SIMD reduction

Use HVecReplicateScalar instead of HVecSetScalars when creating an
initial vector for min/max. This way we are preventing that zeroes
from the initial vector are taken into account for min/max
calculations. Otherwise, min(MAX_INT, x[0],.., x[n-1]) = 0 if each
x[i] is positive which is incorrect.

Added regression test cases in 661-checker-simd-reduc.

Test: ./testrunner.py --target --optimizing in QEMU (arm64)

Change-Id: I1779eefc7f2ab9971dec561b2e1fbf262652410e