cranelift: Add shifts and extends to fuzzer (#4700)

* cranelift: Remove shifts-small-types runtests

These were moved to the main shifts file in #4519 but this file was accidentaly left in tree.

It also fixes the missing sshr_i8_i8 testcase

* cranelift: Add shifts to fuzzer

* cranelift: Add extends to fuzzer

cranelift/filetests/filetests/runtests/shifts.clif

@@ -627,15 +627,15 @@ block0(v0: i8, v1: i16):
 ; run: %sshr_i8_i16(0x40, 33) == 0x20
 ; run: %sshr_i8_i16(0x40, 34) == 0x10
 
-function %sshr_i8_i8(i16, i8) -> i16 {
-block0(v0: i16, v1: i8):
-    v2 = sshr.i16 v0, v1
+function %sshr_i8_i8(i8, i8) -> i8 {
+block0(v0: i8, v1: i8):
+    v2 = sshr.i8 v0, v1
     return v2
 }
 ; run: %sshr_i8_i8(0x80, 0) == 0x80
-; run: %sshr_i8_i8(0x80, 1) == 0x40
+; run: %sshr_i8_i8(0x80, 1) == 0xC0
 ; run: %sshr_i8_i8(0xf0, 0) == 0xf0
-; run: %sshr_i8_i8(0xf0, 4) == 0x0f
+; run: %sshr_i8_i8(0xf0, 4) == 0xff
 ; run: %sshr_i8_i8(0x40, 32) == 0x40
 ; run: %sshr_i8_i8(0x40, 33) == 0x20
 ; run: %sshr_i8_i8(0x40, 34) == 0x10