Fix #335: Introduce variable size recipes and remove GPR_SAFE reg classes (#552)

* Rename size to base_size and introduce a compute_size function;

* Add infra to inspect in/outs registers when computing the size of an instruction;

* Remove the GPR_SAFE_DEREF and GPR_ZERO_DEREF_SAFE register classes on x86 (fixes #335);

cranelift/filetests/regalloc/gpr-deref-safe-335.clif

@@ -0,0 +1,44 @@
+test regalloc
+target x86_64
+
+function u0:587() fast {
+ebb0:
+    v97 = iconst.i32 0
+    v169 = iconst.i32 0
+    v1729 = iconst.i32 0
+    jump ebb100(v97, v97, v97, v97, v97)
+
+ebb100(v1758: i32, v1784: i32, v1845: i32, v1856: i32, v1870: i32):
+    v1762 = iconst.i32 0
+    v1769 = iconst.i32 0
+    v1774 = iconst.i32 0
+    v1864 = iconst.i32 0
+    v1897 = iconst.i32 0
+    jump ebb102(v1774, v1784, v1845, v1856, v1870, v1758, v1762, v169, v1729, v97, v169, v169, v169, v169)
+
+ebb102(v1785: i32, v1789: i32, v1843: i32, v1854: i32, v1868: i32, v1882: i32, v1890: i32, v1901: i32, v1921: i32, v1933: i32, v2058: i32, v2124: i32, v2236: i32, v2366: i32):
+    v1929 = iconst.i32 0
+    v1943 = iconst.i32 0
+    v1949 = iconst.i32 0
+    jump ebb123(v1897, v1769)
+
+ebb123(v1950: i32, v1979: i32):
+    v1955 = iconst.i32 0
+    brz v1955, ebb125
+    jump ebb122(v1929, v1843, v1864, v2058, v1882, v1897, v1943, v1868, v2124, v1901)
+
+ebb125:
+    v1961 = iadd_imm.i32 v1949, 0
+    v1952 = iconst.i32 0
+    v1962 = iconst.i64 0
+    v1963 = load.i32 v1962
+    brz v1963, ebb123(v1952, v1961)
+    jump ebb127
+
+ebb127:
+    v1966 = iconst.i32 0
+    jump ebb122(v1963, v1966, v1966, v1966, v1966, v1966, v1966, v1966, v1966, v1966)
+
+ebb122(v1967: i32, v1971: i32, v1972: i32, v1978: i32, v2032: i32, v2041: i32, v2053: i32, v2076: i32, v2085: i32, v2096: i32):
+    trap user0
+}