PS: Add some simple SSA tests.

powershell/ql/test/library-tests/ssa/explicit.ps1

@@ -0,0 +1,8 @@
+$glo_a = 42
+$glob_b = $glob_a
+
+function f() {
+    $a = 43
+    $b = $a
+    return $b
+}

powershell/ql/test/library-tests/ssa/parameters.ps1

@@ -0,0 +1,11 @@
+function function-param([int]$n1, [int]$n2) {
+    return $n1 + $n2
+}
+
+function param-block {
+    param(
+        [int]$a,
+        [int]$b
+    )
+    return $a + $b
+}

powershell/ql/test/library-tests/ssa/ssa.expected

@@ -0,0 +1,2 @@
+| explicit.ps1:1:1:8:2 | <uninitialized> glob_a | explicit.ps1:2:11:2:18 | glob_a |
+| explicit.ps1:5:5:5:7 | a | explicit.ps1:5:5:5:7 | a |

powershell/ql/test/library-tests/ssa/ssa.ql

@@ -0,0 +1,4 @@
+import powershell
+import semmle.code.powershell.dataflow.Ssa
+
+query predicate definition(Ssa::Definition def, Variable v) { def.getSourceVariable() = v }