[InstCombine] Fix miscompilation in PR83947 (llvm#83993)

https://github.com/llvm/llvm-project/blob/762f762504967efbe159db5c737154b989afc9bb/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp#L394-L407

Comment from @topperc:
> This transforms assumes the mask is a non-zero splat. We only know its
a splat and not provably all 0s. The mask is a constexpr that includes
the address of the global variable. We can't resolve the constant
expression to an exact value.

Fixes llvm#83947.

llvm/test/Transforms/InstCombine/masked_intrinsics.ll

@@ -292,7 +292,11 @@ entry:
 define void @scatter_nxv4i16_uniform_vals_uniform_ptrs_all_active_mask(ptr %dst, i16 %val) {
 ; CHECK-LABEL: @scatter_nxv4i16_uniform_vals_uniform_ptrs_all_active_mask(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    store i16 [[VAL:%.*]], ptr [[DST:%.*]], align 2
+; CHECK-NEXT:    [[BROADCAST_SPLATINSERT:%.*]] = insertelement <vscale x 4 x ptr> poison, ptr [[DST:%.*]], i64 0
+; CHECK-NEXT:    [[BROADCAST_SPLAT:%.*]] = shufflevector <vscale x 4 x ptr> [[BROADCAST_SPLATINSERT]], <vscale x 4 x ptr> poison, <vscale x 4 x i32> zeroinitializer
+; CHECK-NEXT:    [[BROADCAST_VALUE:%.*]] = insertelement <vscale x 4 x i16> poison, i16 [[VAL:%.*]], i64 0
+; CHECK-NEXT:    [[BROADCAST_SPLATVALUE:%.*]] = shufflevector <vscale x 4 x i16> [[BROADCAST_VALUE]], <vscale x 4 x i16> poison, <vscale x 4 x i32> zeroinitializer
+; CHECK-NEXT:    call void @llvm.masked.scatter.nxv4i16.nxv4p0(<vscale x 4 x i16> [[BROADCAST_SPLATVALUE]], <vscale x 4 x ptr> [[BROADCAST_SPLAT]], i32 2, <vscale x 4 x i1> shufflevector (<vscale x 4 x i1> insertelement (<vscale x 4 x i1> zeroinitializer, i1 true, i32 0), <vscale x 4 x i1> zeroinitializer, <vscale x 4 x i32> zeroinitializer))
 ; CHECK-NEXT:    ret void
 ;
 entry: