Use opt_level instead of is_compressed for encoding optimizations.

Choosing smaller instruction encodings on eg. x86 is an optimization,
rather than a useful discrete setting.

Use "is_compressed" only for ISAs that have an explicit compression feature
that users of the output may to be aware of, such as RISC-V's RVC or
ARM's Thumb-2.

cranelift/filetests/isa/x86/allones_funcaddrs32.cton

@@ -1,6 +1,6 @@
 ; binary emission of 32-bit code.
 test binemit
-set is_compressed
+set opt_level=best
 set allones_funcaddrs
 isa x86 haswell
 

cranelift/filetests/isa/x86/allones_funcaddrs64.cton

@@ -1,7 +1,7 @@
 ; binary emission of 64-bit code.
 test binemit
 set is_64bit
-set is_compressed
+set opt_level=best
 set allones_funcaddrs
 isa x86 haswell
 

cranelift/filetests/isa/x86/baseline_clz_ctz_popcount_encoding.cton

@@ -1,7 +1,6 @@
-
 test binemit
 set is_64bit
-set is_compressed
+set opt_level=best
 isa x86 baseline
 
 ; The binary encodings can be verified with the command:

cranelift/filetests/isa/x86/binary32.cton

@@ -1,6 +1,6 @@
 ; binary emission of x86-32 code.
 test binemit
-set is_compressed
+set opt_level=best
 isa x86 haswell
 
 ; The binary encodings can be verified with the command:

cranelift/filetests/isa/x86/binary64-float.cton

@@ -1,7 +1,7 @@
 ; Binary emission of 64-bit floating point code.
 test binemit
 set is_64bit
-set is_compressed
+set opt_level=best
 isa x86 haswell
 
 ; The binary encodings can be verified with the command:

cranelift/filetests/isa/x86/binary64-pic.cton

@@ -1,7 +1,7 @@
 ; binary emission of 64-bit code.
 test binemit
 set is_64bit
-set is_compressed
+set opt_level=best
 set is_pic
 isa x86 haswell
 

cranelift/filetests/isa/x86/binary64.cton

@@ -1,7 +1,7 @@
 ; binary emission of x86-64 code.
 test binemit
 set is_64bit
-set is_compressed
+set opt_level=best
 isa x86 haswell
 
 ; The binary encodings can be verified with the command:

cranelift/filetests/isa/x86/legalize-call.cton

@@ -1,7 +1,7 @@
 ; Test legalization of a non-colocated call in 64-bit non-PIC mode.
 test legalizer
 set is_64bit
-set is_compressed
+set opt_level=best
 isa x86 haswell
 
 function %call() {

cranelift/filetests/isa/x86/prologue-epilogue.cton

@@ -1,6 +1,6 @@
 test compile
 set is_64bit
-set is_compressed
+set opt_level=best
 set is_pic
 isa x86 haswell