Improve uimm128 parsing

This commit changes 128-bit constant parsing in two ways:
 - it adds the ability to use underscores to separate digits when writing a 128-bit constant in hexadecimal; e.g. `0x00010203...` can now be written as `0x0001_0203_...`
 - it adds a new mechanism for parsing 128-bit constants using integer/float/boolean literals; e.g. `vconst.i32x4 [1 2 3 4]`. Note that currently the controlling type of the instruction dictates how many literals to parse inside the brackets.

cranelift/filetests/filetests/isa/x86/rodata-vconst.clif

@@ -1,13 +0,0 @@
-test rodata
-set enable_simd=true
-set probestack_enabled=false
-target x86_64 haswell
-
-; use baldrdash calling convention here for simplicity (avoids prologue, epilogue)
-function %test_vconst_i32() -> i32x4 baldrdash_system_v {
-ebb0:
-    v0 = vconst.i32x4 0x1234
-    return v0
-}
-
-; sameln: [34, 12, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

cranelift/filetests/filetests/isa/x86/vconst-rodata.clif

@@ -0,0 +1,20 @@
+test rodata
+set enable_simd=true
+set probestack_enabled=false
+target x86_64 haswell
+
+function %test_vconst_i32() -> i32x4 {
+ebb0:
+    v0 = vconst.i32x4 0x1234
+    return v0
+}
+
+; sameln: [34, 12, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
+
+function %test_vconst_b16() -> b16x8 {
+ebb0:
+    v0 = vconst.b16x8 [true false true false true false true true]
+    return v0
+}
+
+; sameln: [1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 1, 0]

cranelift/filetests/filetests/isa/x86/vconst-run.clif

@@ -0,0 +1,21 @@
+test run
+set enable_simd
+
+function %test_vconst_syntax() -> b1 {
+ebb0:
+    v0 = vconst.i32x4 0x00000004_00000003_00000002_00000001     ; build constant using hexadecimal syntax
+    v1 = vconst.i32x4 [1 2 3 4]                                 ; build constant using literal list syntax
+
+    ; verify lane 1 matches
+    v2 = extractlane v0, 1
+    v3 = extractlane v1, 1
+    v4 = icmp eq v3, v2
+
+    ; verify lane 1 has the correct value
+    v5 = icmp_imm eq v3, 2
+
+    v6 = band v4, v5
+    return v6
+}
+
+; run