[AMDGPU] Change handling of s_endpgm's optional operand. NFC.

s_endpgm is a special SOPP instruction in that its operand is optional
and if it is not present then we don't want to print a space after the
mnemonic.

Previously this was handled by defaulting real_name to the mnemonic with
a trailing space, and having s_endpgm override it to be the mnemonic
with no trailing space.

This patch implements a different approach where the separator between
Mnemonic and AsmOperands defaults to a space, but s_endpgm overrides it
to be the empty string.

Differential Revision: https://reviews.llvm.org/D139412