[PowerPC] Update handling of splat loads for v4i32/v4f32/v2i64 to require non-extending loads.

This patch updates how splat loads handled and is an extension of D106555.

Particularly, for v2i64/v4f32/v4i32 types, they are updated to handle only
non-extending loads. For v8i16/v16i8 types, they are updated to handle extending
loads only if the memory VT is the same vector element VT type.

A test case has been added to illustrate a scenario where a PPCISD::LD_SPLAT
node should not be produced. In this test, it depicts the following f64
extending load used in a v2f64 build vector, but the extending load is actually
used in more places other than the build vector (such as in t12 and t16).
```
Type-legalized selection DAG: %bb.0 'test:entry'
SelectionDAG has 20 nodes:
  t0: ch = EntryToken
  t4: i64,ch = CopyFromReg t0, Register:i64 %1
  t6: i64,ch = CopyFromReg t0, Register:i64 %2
  t11: f64,ch = load<(load (s64) from %ir.b, !tbaa !7)> t0, t4, undef:i64
        t16: f64 = fadd t31, t37
      t34: ch = store<(store (s64) into %ir.c, !tbaa !7)> t31:1, t16, t6, undef:i64
    t36: ch = TokenFactor t34, t37:1
    t27: v2f64 = BUILD_VECTOR t37, t37
  t22: ch,glue = CopyToReg t36, Register:v2f64 $v2, t27
      t12: f64 = fadd t11, t37
    t28: ch = store<(store (s64) into %ir.b, !tbaa !7)> t11:1, t12, t4, undef:i64
  t31: f64,ch = load<(load (s64) from %ir.c, !tbaa !7)> t28, t6, undef:i64
    t2: i64,ch = CopyFromReg t0, Register:i64 %0
  t37: f64,ch = load<(load (s32) from %ir.a, !tbaa !3), anyext from f32> t0, t2, undef:i64
  t23: ch = PPCISD::RET_FLAG t22, Register:v2f64 $v2, t22:1
```

Differential Revision: https://reviews.llvm.org/D117803