Remove SSE2 setting for x86

In talking to @sunfishcode, he preferred to avoid the confusion of more ISA predicates by eliminating SSE2. SSE2 was released with the Pentium 4 in 2000 so it is unlikely that current CPUs would have SIMD enabled and not have this feature. I tried to note the SSE2-specific instructions with comments in the code.

cranelift/filetests/filetests/isa/x86/pshufb.clif

@@ -1,6 +1,6 @@
 test binemit
 set enable_simd
-target x86_64 has_sse2=true has_ssse3=true
+target x86_64 has_ssse3=true
 
 function %test_pshufb() {
 ebb0:

cranelift/filetests/filetests/isa/x86/pshufd.clif

@@ -1,6 +1,6 @@
 test binemit
 set enable_simd
-target x86_64 has_sse2=true
+target x86_64
 
 function %test_pshuf() {
 ebb0:

cranelift/filetests/filetests/isa/x86/scalar_to_vector.clif

@@ -1,7 +1,7 @@
 test binemit
 set opt_level=best
 set enable_simd
-target x86_64 has_sse2=true
+target x86_64
 
 function %test_scalar_to_vector_b8() {
 ebb0: