Enhancement of BUILD_VECTOR lowering

[kaz7]

We need to enhance BUILD_VECTOR in order to improve generated code in llvm/llvm-project#64420.

From:

        lea %s1, 256
        lvl %s1
        vbrd %v0, 0
        or %s1, 4, (0)1
        lvl %s1
        vstl %v0, 4, %s0
        b.l.t (, %s10)

To:

        or %s1, 4, (0)1
        lvl %s1
        vbrd %v0, 0
        vstl %v0, 4, %s0
        b.l.t (, %s10)