[llvm-exegesis] Avoid yaml parser from calling sscanf for obvious non-matches (PR39102)

[llvm-exegesis] Avoid yaml parser from calling sscanf for obvious non-matches (PR39102)

deserializeMCOperand - ensure that we at least match the first character of the sscanf pattern before calling

This reduces llvm-exegesis uops analysis of the instructions supported from btver2 from 5m13s to 2m1s on debug builds.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@343690 91177308-0d34-0410-b5e6-96231b3b80d8