[AArch64] Fix scalar imm variants of SIMD shift left instructions

This issue was reported in PR50057: Cannot select:
t10: i64 = AArch64ISD::VSHL t2, Constant:i32<2>

Shift intrinsics (llvm.aarch64.neon.ushl.i64 and sshl) with a constant
shift operand are lowered into AArch64ISD::VSHL in tryCombineShiftImm.
VSHL has i64 and v1i64 patterns for a right shift, but only v1i64 for
a left shift.

This patch adds the missing i64 pattern for AArch64ISD::VSHL, and LIT
tests to cover scalar variants (i64 and v1i64) of all shift
intrinsics (only ushl and sshl cases fail without the patch, others
were just not covered).

Differential Revision: https://reviews.llvm.org/D101580

Author: asavonic

llvm/lib/Target/AArch64/AArch64InstrFormats.td

@@ -8958,10 +8958,13 @@ multiclass SIMDScalarLShiftD<bit U, bits<5> opc, string asm,
                              SDPatternOperator OpNode> {
   def d : BaseSIMDScalarShift<U, opc, {1,?,?,?,?,?,?},
                               FPR64, FPR64, vecshiftL64, asm,
-    [(set (v1i64 FPR64:$Rd),
-       (OpNode (v1i64 FPR64:$Rn), (i32 vecshiftL64:$imm)))]> {
+    [(set (i64 FPR64:$Rd),
+       (OpNode (i64 FPR64:$Rn), (i32 vecshiftL64:$imm)))]> {
     let Inst{21-16} = imm{5-0};
   }
+
+  def : Pat<(v1i64 (OpNode (v1i64 FPR64:$Rn), (i32 vecshiftL64:$imm))),
+            (!cast<Instruction>(NAME # "d") FPR64:$Rn, vecshiftL64:$imm)>;
 }
 
 let mayStore = 0, mayLoad = 0, hasSideEffects = 0 in