[TableGen] Give the option of tolerating duplicate register names

A number of architectures re-use the same register names (e.g. for both 32-bit 
FPRs and 64-bit FPRs). They are currently unable to use the tablegen'erated 
MatchRegisterName and MatchRegisterAltName, as tablegen (when built with 
asserts enabled) will fail.

When the AllowDuplicateRegisterNames in AsmParser is set, duplicated register 
names will be tolerated. A backend can then coerce registers to the desired 
register class by (for instance) implementing validateTargetOperandClass.

At least the in-tree Sparc backend could benefit from this, as does RISC-V 
(single and double precision floating point registers).

Differential Revision: https://reviews.llvm.org/D39845

llvm-svn: 320018