[IR2VEC] Rewrite triples.mir and entities.mir. NFC

These test change and need updating every time a new instruction is added,
including generic G_ global isel instructions. The opcodes of all instructions
change force a manual update of the file, which makes it very high cost for
what it is testing.

I have rewritten it to use regexes for the triplets.mir test and just test a
couple of opcodes in the entities.mir file.

Also change them from REQUIRES: x86_64-linux to REQUIRES:
x86-registered-target, as they should run fine on other targets providing that
the triple is used correctly. It passes successfully on an AArch64 box I tried
it on.

Author: davemgreen

llvm/test/tools/llvm-ir2vec/entities.mir

@@ -1,6 +1,8 @@
-# REQUIRES: x86_64-linux
-# RUN: llvm-ir2vec entities --mode=mir %s -o 2>&1 %t1.log
-# RUN: diff %S/output/reference_x86_entities.txt %t1.log
+# REQUIRES: x86-registered-target
+# RUN: llvm-ir2vec entities --mode=mir %s -o - | FileCheck %s
+
+# CHECK: AAA	0
+# CHECK: VirtReg_TILE	{{[0-9]+}}
 
 --- |
   target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"