[NFC] [PowerPC] Remove dead code in BUILD_VECTOR peephole

The piece of code tries to use splat+shift to lower build_vector with
repeating bit pattern. And immediate field of vector splat is only 5
bits (-16~15). It iterates over them one by one to find which
shifts/rotates to number in build_vector.

This patch removes code to try matching constant with algebraic
right-shift because that's meaningless - any negative number's algebraic
right-shift won't produce result smaller than itself. Besides, code
(int)((unsigned)i >> j) means logical shift-right in C.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D93937

llvm/lib/Target/PowerPC/PPCISelLowering.cpp

@@ -9555,17 +9555,6 @@ SDValue PPCTargetLowering::LowerBUILD_VECTOR(SDValue Op,
       return DAG.getNode(ISD::BITCAST, dl, Op.getValueType(), Res);
     }
 
-    // vsplti + sra self.
-    if (SextVal == (int)((unsigned)i >> TypeShiftAmt)) {
-      SDValue Res = getCanonicalConstSplat(i, SplatSize, MVT::Other, DAG, dl);
-      static const unsigned IIDs[] = { // Intrinsic to use for each size.
-        Intrinsic::ppc_altivec_vsrab, Intrinsic::ppc_altivec_vsrah, 0,
-        Intrinsic::ppc_altivec_vsraw
-      };
-      Res = BuildIntrinsicOp(IIDs[SplatSize-1], Res, Res, DAG, dl);
-      return DAG.getNode(ISD::BITCAST, dl, Op.getValueType(), Res);
-    }
-
     // vsplti + rol self.
     if (SextVal == (int)(((unsigned)i << TypeShiftAmt) |
                          ((unsigned)i >> (SplatBitSize-TypeShiftAmt)))) {