[llvm-mca] Clear the content of map VariantDescriptors in InstrBuilder before we...

[llvm-mca] Clear the content of map VariantDescriptors in InstrBuilder before we start analyzing a new CodeBlock. NFCI.

Different CodeBlocks don't overlap. The same MCInst cannot appear in more than
one code block because all blocks are instantiated before the simulation is run.

We should always clear the content of map VariantDescriptors before every
simulation, since VariantDescriptors cannot possibly store useful information
for the next blocks. It is also "safer" to clear its content because `MCInst*`
is used as the key type for map VariantDescriptors.

llvm-svn: 336142