fix: promote mismatched comparison operands before emitting cmp (issue 0146)

A comparison with int-vs-float (or two float widths) operands emitted cmp on
the raw operands with no promotion, unlike the arithmetic arms -- producing a
mixed-type compare the LLVM verifier rejects / mis-evaluates. lowerBinaryOp now
coerces each operand to the promoted common type (from arithResultType) via
coerceToType (SIToFP / FPExt) for the ordering/equality arms when the promoted
type is a float, so LLVM gets a well-typed fcmp.

Regression: examples/0189-types-int-float-compare-promote.sx

examples/0189-types-int-float-compare-promote.sx

@@ -0,0 +1,32 @@
+// A comparison with mismatched scalar operands (int vs float, or two float
+// widths) must promote both operands to the common type before emitting the
+// compare — `lowerBinaryOp` now coerces each operand to the promoted float
+// type for the ordering/equality arms (as the arithmetic arms already do),
+// so the cast materializes and LLVM gets a well-typed fcmp instead of a
+// mixed-type compare that the verifier rejects / silently mis-evaluates.
+//
+// Regression (issue 0146).
+#import "modules/std.sx";
+
+// i32 < f32: the `xx i` cast must materialize as an i32->f32 SIToFP.
+ceil_half :: (v: f32) -> i32 {
+    t := v - 0.5;
+    i : i32 = xx t;
+    if xx i < t {        // 1.0 < 1.8 -> true
+        i += 1;
+    }
+    i
+}
+
+// f64 vs f32 sibling: `xx y + 0.5` infers f64; comparing to an f32 must FPExt.
+ge :: (y: i32, lo: f32) -> i32 {
+    sy := xx y + 0.5;    // 3.5 (f64)
+    if sy >= lo { return 1; }
+    0
+}
+
+main :: () -> i32 {
+    print("{}\n", ceil_half(2.3));   // 2
+    print("{}\n", ge(3, 2.0));       // 1
+    0
+}

examples/expected/0189-types-int-float-compare-promote.exit

@@ -0,0 +1 @@
+0

examples/expected/0189-types-int-float-compare-promote.stderr

@@ -0,0 +1 @@
+

examples/expected/0189-types-int-float-compare-promote.stdout

@@ -0,0 +1,2 @@
+2
+1